Reply: Following the Book of Allah and the Sunnah of the Prophet (S), the Shi‘ah infer religious laws from four fundamental sources:
1. The Book of Allah (Qur’an);
2. The Sunnah of the Prophet (S);
3. Consensus {ijma‘}; and
4. Reason {‘aql}.
Now, we will take a glance at the Book of Allah and the Sunnah of the Prophet (S) which are the most fundamental fountainheads of Shi‘i jurisprudence {fiqh}:

1) The Book of Allah (Qur’an)
The followers of the Shi‘i school regard the Qur’an as the firmest source of its jurisprudence and the criterion of understanding divine laws. They consider the heavenly book of Islam as the loftiest reference of divine laws, and think that any opinion must be assessed according to the Qur’an, and in case of its conformity with the Qur’an, it is accepted, and if not, it is rejected.
Imam as-Sadiq (‘a), the sixth Imam followed by the Shi‘ah, thus says in this regard:

"وكل حديث لا يوافق كتاب الله فهو زخرف."

“Any statement, which is not in conformity with the Book of Allah, is worthless.”1
Also, Imam as-Sadiq (‘a) thus narrates from the Holy Prophet (S):

"أيها الناس ما جاءكم عني يوافق كتاب الله فأنا قلته وما جاءكم يخالف كتاب الله فلم أقلة."

“O people! When you hear a statement which is attributed to me and is in harmony with the Book of Allah, take it for granted that I have said it; otherwise, I have not said it.”2
These two hadiths show clearly that leaders of the Shi‘ah regard the holy scripture of Muslims as the firmest source of religious laws.

2) The Sunnah
The Sunnah, that is the sayings, actions of the Messenger of Allah (S) and whatever he approved, is the second fountainhead of Shi‘i jurisprudence, and the Imams from the Ahl al-Bayt of the Prophet (S) are independently regarded as the transmitters of the Sunnah of the Prophet (S) and repository of his knowledge. Of course, the Shi‘ah also accept the Prophetic hadiths which are transmitted by reliable people other than the Imams (‘a).
At this point, it is appropriate to consider two things:

The reason for clinging to the Sunnah of the Prophet (S)
The leaders of the Shi‘ah have enjoined their followers to cling to the Sunnah of the Prophet (S) alongside the Qur’an, commending both the Book of Allah and the Sunnah of the Prophet (S). Imam as-Sadiq (‘a) says:

"إذا ورد عليكم حديث فوجدتم له شاهداً من كتاب الله أو من قول رسول الله (ص) إلا فالذي جاءكم به أولى به."

If you are introduced to a narration which conforms with the Book of Allah or the words of the Messenger of Allah (S), accept it; otherwise, that narration is more worthy for its transmitter.3
Similarly, Imam al-Baqir (‘a) considers holding fast to the Sunnah of the Prophet (S) as a basic characteristic of an extremely well qualified jurist, and says:

"إن الفقيه حق الفقيه الزاهد في الدنيا الراغب في الأخرة المتمسك بسنّة النبيّ (ص)."

“A true jurist is he who renounces what is in this world, desires for what is in the hereafter and clings to the Sunnah of the Prophet (S).”4
Great leaders of the Shi‘ah have such a good mastery over the Sunnah that they reject whatever opposes the Book of Allah and the Sunnah of the Prophet (S), and consider it as kufr {denial of faith}. Imam as-Sadiq (‘a), who stresses this idea, says:

"من خالف كتاب الله وسنّة محمد (ص) فقد كفر."

“Anyone who opposes the Book of Allah and the Sunnah of Muhammad (S) is an infidel.”5
This shows clearly that the Shi‘ah observe the Sunnah of the Holy Prophet (S) more than any other Muslim groups, and testify to the groundlessness of the statement of those who accuse the Shi‘ah of being alien to the Sunnah of the Prophet (S).

The reason for clinging to the hadiths of the Ahl al-Bayt (‘a)
In order to clarify the Shi‘ah’s view regarding the hadiths of the Prophet’s progeny (‘a), two topics are worth considering:
1. The essence of the hadiths of the infallible Imams (‘a), and
2. The proofs of the credibility and necessity of holding fast to the Ahl al-Bayt of the Prophet (S).
Having provided solid proofs, we shall examine the two topics briefly:

The essence of the hadiths of the progeny of the Messenger of Allah (S)
According to the Shi‘ah, only the Lord of the worlds has the right to legislate for human society. He disseminates sacred religious laws and ordinances to the world through His Prophet (S). It is obvious that the Messenger of Allah (S) is the only one who receives revelation and religious instructions from God and imparts them to the people.
Therefore, it is clear that when the Shi‘ah regard the hadiths of the Ahl al-Bayt (‘a) as a source of their fiqh, this does not mean that they consider these hadiths as independent of the Sunnah of the Holy Prophet (S) but they ascribe the credibility of the hadiths of the Prophet’s progeny (‘a) to the fact that they clarify the meaning of the Sunnah of the Messenger of Allah (S).
Therefore, the hadiths of the infallible Imams followed by the Shi‘ah are not of their own; whatever they say is the same as the Sunnah of the Holy Prophet (S).
In order to prove this idea, it is appropriate to quote some of the narrations of the Prophet’s progeny (‘a):
1. In response to a man’s question, Imam as-Sadiq (‘a) says:

"مهما أجبتك فيه بشئ فهو عن رسول الله (ص) لسنا نقول برأينا من شئ."

“All the answers I give you are based on the words of the Messenger of Allah (S) and we do not say anything of our own.”6
He (‘a) also says elsewhere:

حديثي حديث أبي وحديث أبي حديث جدّي وحديث جدّي حديث الحسين وحديث الحسين حديث الحسن وحديث الحسين حديث أمير المؤمنين وحديث أمير المؤمنين حديث رسول الله (ص) وحديث رسول الله قول الله عزّوجلّ.

My hadith is my father’s hadith, and my father’s hadith is my grandfather’s hadith, and my grandfather’s hadith is al-Husayn (‘a)’s hadith, and al-Husayn’s hadith is al-Hasan’s hadith, and al-Hasan’s hadith is the Commander of the Faithful’s hadith, and the Commander of the Faithful’s hadith is the Messenger of Allah’s hadith (S), and the Messenger of Allah’s hadith is from Allah, the Honorable and Glorious.7
2. Imam Muhammad al-Baqir (‘a) thus says to Jabir (ibn ‘Abd Allah al-Ansari):

"حدثني أبي عن جدّي رسول الله (ص) عن جبرئيل عن الله عز وجل وكلما أحدثك بهذا الإسناد.

My father narrated to me from the Messenger of Allah (S), and he from Jibra’il (Archangel Gabriel) (‘a), and Jibra’il from God, the Honorable and Glorious, and all my narrations are based on this chain of transmission.8
The mentioned hadiths show clearly that the hadiths of the Imams followed by the Shi‘ah are the very Sunnah of the Holy Prophet (S).

The proofs of the credibility and necessity of clinging to the Ahl al-Bayt (‘a)
The hadith scholars {muhaddithun} of both Sunni and Shi‘ah schools of thought are of the opinion that the Messenger of Allah (S) has left behind two precious legacies and invited all Muslims to hold fast to these two legacies, regarding that felicity and guidance are the consequence of people’s holding fast to these two - the first is the Book of Allah (the Qur’an) and the second is his progeny {‘itrah} and the members of his Household {Ahl al-Bayt} (‘a).
Here are some examples of these traditions:
1. In his Sahih (or Sunan), Tirmidhi thus narrates on the authority of Jabir ibn ‘Abd Allah al-Ansari that the Messenger of Allah said:

"يا أيها الناس إني قد تركت فيكم ما إن أخذتم به لن تضلوا: كتاب الله وعترتي أهل بيتي."

“O people! I am leaving behind two things, which if you hold fast to, you will never go astray: the Book of Allah and my progeny, the members of my Household.”9
2. Tirmidhi thus also writes in the mentioned book:

قال رسول الله صلى الله عليه وأله وسلم: إني تارك فيكم ما إن تمسكتم به لن تضلوا بعدي أحدهما أعظم من الأخر: كتاب الله حبل ممدود من السماء إلى الأرض وعترتي أهل بيتي لن يفترقا حتى يردا عليّ الحوض فانظروا كيف تخلفوني فيهما.

The Messenger of Allah (S) said: Verily, I am leaving among you two weighty things, which if you hold fast to, you will never go astray; one is greater than the other: the Book of Allah, which is a cord extending from the heaven to earth. The other is my progeny, the members of my Household. These two will never separate from each other until they meet me at the Pond {hawd} (of Kawthar). Be careful how you will behave with them when I leave you.10
3. In his Sahih, Muslim ibn al-Hajjaj narrates from the Holy Prophet (S) and says:

ألا أيها الناس فانما أنا بشر يوشك أن يأتي رسول ربي فأجيب وأنا تارك فيكم ثقلين: أولهما كتاب الله فيه الهادي والنور فخذوا بكتاب الله واستمسكوا به – فحث على كتاب الله ورغب فيه ثم قال: وأهل بيتي أذكركم الله في أهل بيني أذكركم الله في أهل بيتي أذكركم الله في أهل بيتي.

“O people, I am a human being. I am about to receive a messenger (the angel of death) from my Lord and I, in response to Allah’s call, (would bid goodbye to you), but I am leaving among you two weighty things: the one being the Book of Allah in which there is right guidance and light, so hold fast to the Book of Allah and adhere to it.” He exhorted (us) (to hold fast) to the Book of Allah and then said: “The second are the members of my Household. I remind you (of your duties) to the members of my family.”11
4. A group of hadith scholars has reported the Holy Prophet (S) to have said:

إني تارك فيكم الثقلين كتاب الله وأهل بيتي وإنهما لن يفترقا حتى يردا عليّ الحوض.

I am leaving among you two weighty things {thaqalayn}: the Book of Allah (the Qur’an) and my Household {ahla bayti}, and these two will never be separated from each other until they meet me at the Pool {hawd} (of Kawthar).12
It is necessary to note that the hadiths related to this topic are too many to be covered in this volume, and the prolific researcher, Sayyid Mir Hamid al-Husayn has compiled the chains of transmission of these traditions in his six-volume ‘Abaqat al-Anwar.
These quoted traditions clearly show that clinging to and following the Ahl al-Bayt of the Prophet (S), alongside the Book of Allah and the Sunnah of the Holy Apostle (S), is among the exigencies of Islam, and ignoring the hadiths of the Prophet’s progeny (‘a) leads to misguidance and loss.
Now, who are the progeny of the Prophet (S) whose obedience is incumbent upon us by the order of the Messenger of Allah (S)? In order to clarify this point, we shall cite some narrations and examine the meaning of “the progeny {‘itrah} of the Prophet (S)”:

Who are the Ahl al-Bayt of the Prophet (S)?
The quoted traditions affirm that the Holy Prophet (S) called upon all Muslims to follow his progeny, considering them, alongside the Book of Allah, as the authorities for people to refer to after his departure. He said very clearly: “The Qur’an and my progeny will never be separated from each other.”
Now, because the Ahl al-Bayt of the Prophet (S) who are introduced by the Messenger of Allah (S) as equal to the Qur’an, we realize that they enjoy the station of infallibility {‘ismah} and they have access to the pure fountain of Islamic knowledge; for, if this is not the case, they will be separated from the Book of Allah (the Qur’an), whereas the Holy Prophet (S) says: “The Qur’an and my progeny will never be separated from each other until they meet me at the Pool {hawd} (of Kawthar).”
As such, it is necessary to fully recognize the Ahl al-Bayt (‘a) and their outstanding attributes, which characterize only the Imams followed by the Shi‘ah, all of whom are among the progeny of the Prophet (S).
Referring to the traditions transmitted by great muhaddithun, we shall bring forth our convincing proofs in this regard:
1. After narrating the Hadith ath-Thaqalayn, Muslim ibn al-Hajjaj thus says:
Yazid ibn Hayyam asked Zayd ibn al-Arqam: “Who are the members of the Household of the Prophet (S)? Aren’t the wives (of the Holy Prophet) included among the members of his household?” In reply, Zayd ibn Arqam thus said:

"لا وايم الله إن المرأة تكون مع الرجل العصر من الدهر ثم يطلقها فترجع إلى أبيه وقومها أهل بيته أصله وعصبته الذين حُرموا الصدقة بعده."

No, by Allah, a woman lives with a man (as his wife) for a certain period; he then divorces her and she goes back to her parents and to her people; the members of his household include his own self and his kith and kin (who are related to him by blood) and for him the acceptance of zakah is prohibited.13
This tradition testifies to the fact that “the “progeny of the Prophet (S),” clinging to whom, like holding fast to the Qur’an, is obligatory, does not mean his wives but they, apart from the close physical affinity and spiritual attachment they have with him, have a special merit for which we consider alongside the Qur’an, authorities for the Muslims of the world to refer to.
2. The Holy Prophet (S) did not only describe the attributes of the Ahl al-Bayt (‘a) but also mentioned their number, and said that they are twelve:
Muslim narrates on the authority of Jabir ibn Samurah:

سمعت رسول الله صلى الله عليه وأله وسلم يقول: لا يزال الإسلام عزيزاً إلى إثنى عشر خليفة ثم قال كلمة لم اسمعها فقلت لأبي ما قال؟ فقال: كلهم من قريش."

I heard the Messenger of Allah (S) say: ‘Islam will keep its honor with twelve caliphs.’ Then, he said a statement which I did not hear. I asked my father, “What he (S) said?” Then, he (my father) replied that he (S) said: “All of them will be from Quraysh.14
Also, Muslim ibn al-Hajjaj thus narrates from the Messenger of Allah (S):

"لا يزال أمر الناس ماضياً ما وليهم إثنا عشر رجلاُ."

“The affairs of the people will continue to be conducted (well) as long as they are governed by twelve men.”15
The two traditions are a clear testimony to the Shi‘ah contention that “The twelve Imams followed by the Shi‘ah are the rightful leaders of the people after the Holy Prophet (S).” This is because in Islam, the twelve caliphs who came immediately after the Messenger of Allah (S) are the authorities who take care of the Muslims’ affairs and the glory and splendor of Islam, are referred to no one except the twelve Imams from the Ahl al-Bayt (‘a).
For, if we apply that to the four caliphs who are known as the “Rightly-Guided Caliphs” {khulafa ar-rashidun}, we find that the other rulers—the Umayyad and ‘Abbasid caliphs—most of whom are known for their unscrupulous characters as testified by history, are a source of Islam and the Muslims’ ignominy.
As such, the “Ahl al-Bayt”, who have been introduced by the Holy Prophet (S) as equal to the Qur’an and as the authority for the Muslims in the world to refer to, are the very twelve leaders from the Prophet’s progeny. They are the preservers of the Sunnah of the Messenger of Allah (S) and the repository of his knowledge.
3. Also, the Commander of the Faithful, ‘Ali ibn Abi Talib (‘a), says that the leaders of Muslims are from Banu Hashim, which is another clear testimony to the soundtness of the Shi‘ah premise in their recognition of the Ahl al-Bayt (‘a), when he declares:

"إن الأئمة من قريش غرسوا في هذا البطن من بني هائم لا تصلح على من سواهم ولا تصلح الولاة من غيرهم."

Surely, the Imams (divine leaders) will be from the Quraysh. They have been planted in this line through Hashim. It would not suit others nor would others be suitable as heads of affairs.16

Conclusion
The set of the quoted of traditions reveals two facts:
1. Holding fast to the Ahl al-Bayt of the Prophet (S) and following them alongside the injunctions of the Book of Allah is obligatory.
2. The Ahl al-Bayt of the Messenger of Allah (S) who have been introduced as “the counterpart of the Glorious Qur’an” and as the authority for all Muslims to refer to, have the following salient features:
a. All of them are from the tribe of Quraysh and the clan of Banu Hashim;
b. They have such affinity to the Messenger of Allah (S) that charity {sadaqah} is unlawful for them to receive;
c. They have the station of infallibility {‘ismah}, otherwise they would be separated from the Glorious Qur’an, whereas the Holy Prophet says: “These two (the Qur’an and ‘itrah) will not be separated from each other until they meet me at the Pool {hawd} of Kawthar.”
d. They are twelve all in all and they are the guardians and leaders of the Muslims who succeed the Messenger of Allah (S) one after the other.
e. These twelve successors of the Prophet (S) are the source of ever-growing glory and splendor of Islam.
Taking into account these descriptions drawn from the traditions, we realize that by his Ahl al-Bayt (‘a), the obedience to whom is incumbent on the Muslims, the Prophet (S) means the very twelve pure Imams from the progeny of the Holy Prophet (S), obedience and loyalty to whom the Shi‘ah take pride when exploring their jurisprudential laws.

Notes
1. Usul al-Kafi, vol. 1, “Kitab Fadl al-‘Ilm,” “Bab al-Akhdh bi’s-Sunnah wa Shawahid al-Kitab,” hadith 3.
2. Ibid., hadith 5.
3. Ibid., hadith 2.
4. Ibid., hadith 8.
5. Ibid., hadith 6.
6. Jami‘ Ahadith ash-Shi‘ah, vol. 1, p. 129.
7. Ibid., p. 127.
8. Ibid., p. 128.
9. Sahih Tirmidhi (Beirut), “Kitab al-Manaqib,” “Bab Manaqib Ahl Bayt an-Nabi,” vol. 5, p. 662, hadith 3786.
10. Ibid., p. 663, hadith no. 3788.
11. Sahih Muslim (Egypt), vol. 7, “Bab Fada’il ‘Ali ibn Abi Talib, pp. 122-123; Sahih Muslim, vol. 4, p. 1803, hadith no. 2408 (‘Abd al-Baqi Edition).
Abdul-Hamid Siddiqui (trans.), Sahih Muslim (English Translation), vol. 4, hadith no. 5920. {Trans.}
12. Mustadrak al-Hakim, vol. 3, p. 148; As-Sawa’iq al-Muhriqah, sec. 11, chap. 1, p. 149, and a similar narration is also presented in the following books: Musnad Ahmad ibn Hanbal, vol. 5, pp. 182, 189; Kanz al-‘Ummal, vol. 1, “Bab al-I‘tisam bi’l-Kitab wa’s-Sunnah,” p. 44.
13. Sahih Muslim (Egypt), vol. 7, “Bab Fada’il ‘Ali ibn Abi Talib, p. 123.
Abdul-Hamid Siddiqui (trans.), Sahih Muslim (English Translation), vol. 4, book, 31, hadith no. 5923. {Trans.}
14. Muslim, Sahih (Egypt), vol. 6, p. 2.
Abdul-Hamid Siddiqui (trans.), Sahih Muslim (English Translation), vol. 3, hadith no. 4480. {Trans.}
15. Ibid.
Abdul-Hamid Siddiqui (trans.), Sahih Muslim (English Translation), vol. 3, hadith no. 4478. {Trans.}
16. Nahj al-Balaghah (Subhi Salih), Sermon 144.