Some people ask why shia pray Namaze Zohr and Namaze Asr together, Namaze Maghrib and Namaze Isha together, while majority of Muslims are praying these Namazes separately and the name of every Namaz is different?
To reply to this question, we should put forward our principles in all our religious matters that we are following what exactly the Prophet Mohammed (S.A.W.) was doing. In our Namaz, how to pray Namaz, the timings of Namaz, and preparation for Namaz e.g. Wazu etc. and all things concerned with Namaz, we take it from the Prophet (S.A.W.) through the most authentic source which is Ahlul Bayt (A.S.) Not only Namaz but also all other Ahkaam (Rules). Every Hokm (Rule) in the Sharish we take from the Quran and from the Prophet (S.A.W).
Obviously, all Muslim sects inspite of their differences, are claiming to be following Quran and Sunnat. But simply claiming without sufficient evidences does not prove any fact. We have full and complete proofs that the Sunnat which we follow is the real and original and pure Sunnat, being conveyed to us by the topmost authentic personalities i.e., Ahlul Bayt (A.S.) who lived with the Prophet and knew each and everything about his sayings and his deeds.
The interpretation and understanding of Quran depends also on Hadeeths. Only authentic Hadeeth can be considered in this regard i.e., Real and pure Sunnat is the only way, to understand Quran. As a result, we clearly observe that real and pure Sunnat, which we got through Ahlul Bayt (A.S.), is the only way to follow Quran and the Prophet (S.A.W.).

How to pray Namaz?
Real Muslim should follow the Prophet (S.A.W.) in all details of his faith, worship, deeds and morals.
But because of the special importance of Namaz being the Pillar of Deen, the Prophet (S.A.W.) ordered all Muslims strictly to pray exactly as he was praying.
It means clearly that.
(A) If the Prophet (S.A.W.) was doing something regularly all times in his Namaaz, that thing becomes Wajib e.q: reciting Sure Al-Hamd in the first two Rakaats. No Muslim can leave deliberately this Wajib being compulsory.
(B) If the Prophet (S.A.W.) never did something while praying. Namaz that it is Haraam e.g. praying Namaze Asr and Namaze Maghrib together which was never done by the Prophet, i.e. it is Haraam.
(C) If the Prophet (S.A.W.) was doing something sometimes, it means that things is allowed and permissible (Jaez) e.g. praying at Masjid or at home, reciting long sura or short sura after Al-Hamd in the first two rakaats. Praying Zohr and Asr together, or Maghrib and Isha together is another example of Namaz, which the prophet (S.A.W.) was doing many times, simply and willingly and purposely without any extraordinary circumstances such as rain or travelling or war, or Hajj etc.
We are praying Namaz together simply because all proofs and all evidences clearly show that the Prophet (S.A.W.) was praying Zohar and Asr together, and Maghrib and Isha together. We do not claim that he was always praying together, otherwise it would have been Wajib to pray together, but since he was praying together many times, and without any extraordinary circumstances, it means that it is permissible (Jaez). It also means that it is Sunnat-e-Rasool (S.A.W.). We do not believe that it is compulsory (Wajib) to pray together always, because Prophet (S.A.W.) was praying sometimes separately. But we wonder how could some Muslims claim praying together as Haraam, inspite of having authentic proofs that the Prophet himself was doing it. When the Prophet (S.A.W.) himself was doing it, it means that it is Sunnat, being his act, then by believing and acting on this way of praying Zohar and Asr together we are really following the Prophet and implementing his Sunnat, because we believe it Wajib and must not only to follow him in this point but also in all the other parts of Shariat.

Namaz Timing in Quran
We would like to refer to the Holy Quran and see the timings of Namaz which were given from Allah Ta'ala to the Muslims.
First Ayat: Ayat No. 78 in sure Bani Israil ALLAH, the Most Exalteds says:
"Keep up prayers when declining of sun, till the darkness of the night, and the morning recitation; surely the morning recitation is witnessed".
In this Ayat, Allah Ta'ala clearly gives the timings of Namaz as follows.
1. When declining of Sun: Which is the beginning time of Namaze Zohr and Namaze Asr.
2. The darkness of the night: Which is the beginning of Namaze Maghrib and Namaze Isha.
It is useful to state that the timing of Maghrib Namaz and Iftar of fasting differ between Shias who are followers of Real Islam through Ahlul bayt (A.S.) and other Muslims who mainly follow either Abu Hanifa, Shaafeie, Mealik, or Ibne Hambal. Shia Muslims believe that time of Maqhrib Namaaz start after vanishing of the reddish light in the west, which is reflection of sunlight when sun is unseen in east. This again proves that time of Maghrib Namaze is beginning of the night, which exactly is the time considered by Shia Muslims.
3. The Morning recitation: Which is the time of morning prayers. It is mentioned in the Ahadith (traditions) that (Morning recitation is surely being witnessed) because the Angels of Night along with Angels of Day, both witness the Morning Prayer.
It is clear from this Ayat that main timing for Namaz are three,
First: Zohar and Asr
Second: Maghrib and Isha
Third: Morning
Let us see the second Ayat: Ayat No. 114 in Sure Hood ALLAH, the Most Exalteds says:
"And keep up prayer in the two sides of the day and in the beginning of the night, surely good deeds take away evil deeds. That is a reminder to the mindful".
In this, Ayat timings of Namaz are given again clearly as Three main times, as follows:
First & Second: The sides of the day; which means morning which is the beginning side of the day, and Afternoon which is the other side of the day, when Zohr and Asr Namaz time starts.
Third:In the beginning of the night; which is the time of Magrib and Isha Namaz.
It is then very clear that timing of Namaz in Quran, are three main items for five prayers.
First: From Afternoon unto before sunset: for Namaze Zohr then Namaze Asr.
Second: From Real Sunset when Night starts beings the time for Namaze Maghrib and Namaze Isha.
Third: Morning Namaz.

Timings of Namaz of the Prophet (S.A.W.)
All muslims believe in the principle of combining timings of Zohar and Asr, and Maghrib and Isha, but Non Shia claim that this combining is allowed in certain circumstances such as rainy climate, travelling specially while in pilgrimage (Arafat) or in illness, or war, while Shia Muslims give strong evidences that the Prophet (S.A.W.) was combining the Namaz without these restrictions. The important Books of Traditions e.g: Al-kaafi, Al-Faqeeh, Al-Estibsar, and Al-Tahzeeb, contain big number of Traditions from the prophet (S.A.W.) and Ahlul Bayt (A.S.) that the Prophet (S.A.W.) was praying Zohar and Asr, and Maghrib and Isha together in the ordinary and normal days. But we will not refer to these books because they are Shia books and we want to prove the fact to the Non-Shia, far that we will prove it from their own books which are considered by them as the main source of Shariat after Quran.
First Hadeeth: - In Saheeh Muslim - which is considered by Sunnis, along with Saheeh Bukhari, as most authentic books after Quran - In the Chapter, of (combining two Prayers)
Ibn Abbas said - that the Prophet (S.A.W.) prayed Zohr and Asr in Medina together without any cause for fear and without travelling. Then Ibn Abbas was asked: Why the Prophet did that. He replied: - He did not want anybody from his Ummah to face difficulty.
Second Hadeeth: - In Saheehul Bukhari - which is the topmost book among Sunnis. The Chapter of delaying Zohr Namaaz upto Asr, from the part of Narmaz timing.
Ibn Abbas said that the Prophet (S.A.W.) prayed in Medina seven Rakaat and Eight Rakat (means Maghrib and Isha seven and Zohr and Asr eight).
Third Hadeeth: - In Saheehul Bukhari from Ibn Abbas that the Prophet prayed Zohr and Asr together and Maghrib and Isha together, without any cause for fear or travelling.
Ibn Abbas was asked why, He replied: The Prophet did not want to cause difficulty far his Ummah.
Fourth Hadeeth: - Al-Tabarani from Abdullah lbn Masoud said that the Prophet (S.A.W.) prayed Zohr and Asr together and Maghrib and Isha together, and when he was asked why he prayed together, he said: - I did this, not to put my Ummah in difficulty. This Hadeeth is mentioned in Sharh Al Mowatta'a by Al-zargani Vol. 1 page 263.
Fifth Hadeeth: - Abdulla Ibn Umar had been asked: Why the Prophet was praying Zohr and Asr together and Maghrib and Isha together without travelling and without cause for fear?
Abdulla Ibn Umar replied: - He did that not to put his Ummah in difficulty. This Hadeeth is mentioned in the famous book Kanz Al-Ommaal Vol. 4 page 242.
Sixth Hadeeth: - In Saheeh Muslim that Ibn Abbas said: - We used to pray two Namaaz together in the time of the Prophet (S.A.W.).
It is to be mentioned that Muslim in his Saheeh had kept special chapter for (combining two Namaz without travel) in which he had mentioned more than ten authentic Hadeeths proving that the prophet (S.A.W.) was praying together without any restriction.
The reason of combining of two Namaz is clearly mentioned in the traditions (Ahadeeth) that not to keep the Muslims in difficulty, because separating Zohr from Asr, and Maghrib from Isha, causes sometimes hardships and difficulty, specially, for working and busy people. The difficulty may cause in leaving praying Namaz, as we see among many Sunnis, who cannot easily pray five separate times because it is not practical for them.

Some Important Benefits

Combining two Namaaz is a facility and bounty and gift from Allah Taala to the believers, because He knows that this is the practical way to pray, for many persons and in many circumstances. Nobody has right to close this gate of mercy, which was opened by Allah and practiced many times by the Prophet (S.A.W.) himself.
Combining, two Namaz has got apart from the benefit mentioned, other benefits one of which is shown in the Hadeeth, saying that a man complained to Imam Hasan Askari (A.S.) that he was rich and become poor. Imam (A.S.) told him: - Combine two Namaz, then you will see what you like.
 
Conclusion
From all these evidences from Quran and authentic Ahaadeeth it is obviously proved that Praying Zohr and Asr together, and Maghrib and Isha together is the real Sunnat and way of the Prophet (S.A.W.). Those who deny that are denying real Sunnat, and objecting on a work which was, done by the Prophet himself. Those who are following the real Islam who are called Shia are following the Prophet and practicing whatever he was saying and doing.
 
Courtesy: Hujjatul Islam Agha Sayed Mohammed Al-Musawi

Timings

Hence from the above discussion, we can represent the timings of Namaz in a form of a circle (day & night) as shown above, and the detail timings are given in the below table.
Prayer:
(Al-Fajr) Dawn

Start Time: From dawn when a brightness appears in the eastern horizon at the end of the night.

Specific Time: From dawn and ends with the appearance of the twilight in the eastern horizon.

End Time: When the red rays appear in the eastern horizon.

(Al-Zohr)Noon

Start Time: When the sun is perpendicular slightly over it position or when the sun is travels towards slightly the west over half way in the horizon.

Specific Time: From noon to the time when the shadow of an object becomes equal to its length, which usually takes 15 minutes till the noon prayer performance.

Sharing Time: Between the noon and the after noon specific time.

End Time: End of Sharing time.

(Al-Asr) Afternoon

Start Time: Begins from the time when the specific time for the noon prayer ends.

Specific Time: Just before sunset enough for afternoon prayer performance.

Sharing Time:Between the noon and the after noon specific time.

End Time: To the time of sunset.

(Al-Magrib) Sunset

Start Time: When the sun disappears in the western horizon the eastern sky darken and the western sky turns reddish.

Specific Time: From the sunset start time till the Magrib prayer performance.

Sharing Time: Between the sunset specification time and evening specific time.

End Time: End of sharing time.

(Al-Isha) Evening

Start Time: Starts from the time when the specific time for the sunset prayer ends.

Specific Time: Just before Midnight till midnight time enough for the night prayer performance.

Sharing Time:Between the sunset and the evening specific time.

End Time: Midnight.