The formal declaration of Imamat of Imam Ali (A.S.) at Ghadir Khumm has been unanimously described by the learned historians and scholars of both sects. Here a brief account is given to show what great arrangements were made to declare Imam Ali (A.S.) as the successor to the Holy Prophet (S.A.W.).
Ghadir Khumm lies in Juhfa between Makkah and Madinah. When the Holy Prophet (S.A.W.) was on his way home, after performing his last pilgrimage, Jibrael brought him this urgent command of Allah:
O Apostle! deliver what has been sent down to you from your Lord; and if you do it not, then you have not delivered His message (at all); and Allah will protect you from the people . . . (5 :67)
The Holy Prophet (S.A.W.) stopped at once and ordered that all people who had gone ahead should be called back, and he waited for those who were following. When the entire caravan had gathered, a pulpit was set up by piling up camel saddles; the acacia thorns were swept away.
The Holy Prophet (S.A.W.) ascended the pulpit and delivered a long sermon. The day was very hot; people had to stretch their cloaks under their feet and over their heads.
The Holy Prophet addressed them as follows:
O you people! Know it well that Jibrael came down to me several times bringing me orders from the Lord, the Merciful, that I should halt at this place and inform every man, white and black, that Ali(A.S.), the son of Abu Talib, is my brother and my wasiyy (successor) and my caliph, and the Imam after me. His position to me is like that of Harun to Musa, except that there is to be no prophet after me, and he is your master next to Allah and His Prophet.
O you people! Verily, Allah has appointed him to be your Imam and ruler. Obedience to him is obligatory alike on all the muhajirun (Emigrants) and ansar (Helpers) and on those who follow them in virtue, and on the dwellers in cities and the nomads, the Arabs and the non-Arabs, the freeman and the slave, the young and the old, the great and the small, the white and the black. His command is to be obeyed, his word is binding and his orders obligatory on everyone believing in the One God. Cursed is the man who disobeys him and blessed is he who follows him, and he who believes in him is a true believer.
O you people! This is the last time I shall stand in this assembly. Therefore, listen and obey and surrender to the command of your Lord. Verily, Allah, He is your Lord and God; then after Him, His Prophet, Muhammad(S.A.W.), who is addressing you, is your Master, then after me 'Ali(A.S.) is your Master and your Imam, according to Allah's command.
Then after him the Imamate will continue through my descendants begotten by him till the day you meet Allah and His Prophet. O you people! Meditate on the Qur'an and understand its verses; reflect over its clear verses and do not go to the ambiguous ones. For, by Allah, none will properly explain to you its warnings and expound to you its meanings except , this man (i.e.,'Ali) whose hand I am lifting up in front of myself. And I say unto you that "For whoever I am his Leader (mawla), 'Ali is his Leader (mawla)."
and he is Ali(A.S.), the son of Abu Talib, my brother and wasiyy (successor); and wilayah (obedience to him and love for him) has been made obligatory by Allah, the Powerful, the Exalted.
The other Imams have also been briefly referred to in this address; and they are mentioned in precise detail in many other traditions. For example, on one occasion addressing Imam Husayn(A.S.) the Holy Prophet(S.A.W.) said:
"You are an Imam, the son of an Imam, the brother of an Imam, nine of your lineal descendants will be pious Imams; the ninth of them being their Qa'im (he who will rise).'' [al-Qunduzi: Yanabi'u 'l-mawaddah, [p.168; Amritsari: Arjahu 'l-matalib, p.448].
Even a casual observer would not fail to realize that, it was a matter of vital importance to Islam and that is why the Holy Prophet (S.A.W.), under the Divine Command, made all the possible preparations to accomplish it. Exposed to the scorching rays of the midday sun, he mounted the pulpit to make the important pronouncement.
First of all, he informed the audience of his approaching end and then called them to witness that he had faithfully discharged his duties. Then he asked them:
"Do I not have more authority upon you than you yourselves have."
All of them cried out that he certainly had more right on them than they themselves had. The Holy Prophet (S.A.W.) then said: "Whoever whose Master I am, 'Ali is his Master." In the end he invoked blessings on 'Ali, saying: "O Allah! Love him who loves 'Ali, and be the enemy of the enemy of 'Ali; help him who helps 'Ali, and forsake him who forsakes 'Ali.''
When the ceremony was over, the following verse of the Qur'an was revealed:
This day I have perfected your religion for you and I have completed My bounty upon you and I have approved Islam as your religion (5:3).
This Divine Communication clearly shows that because of 'Ali's appointment to thee Imamate the religion was perfected, the bounty; and favour of Allah completed, and Islam approved by Allah. On the arrival of this glad tiding from heaven the believers congratulated 'Ali in the Prophet's presence and many poets composed poems on this event. All these facts stand recorded in books of tradition as will be seen in the following pages.

Oath of Allegiance
After his speech, the Messenger of Allah(S.A.W.) asked everybody to give the oath of allegiance to Ali(A.S.) and congratulate him. Among those who did so was 'Umar b. al-Khattab, who said: "Well done Ibn Abi Talib! Today you became the Leader (mawla) of all believing men and women."

Number of People in Ghadir Khumm
Allah ordered His Prophet(S.A.W.) to inform the people of this designation at a time of crowded populous so that all could become the narrators of the tradition, while they exceeded a hundred thousand. Narrated by Zayd b. Arqam: Abu al-Tufayl said: "I heard it from the Messenger of Allah (S.A.W.), and there was no one (there) except that he saw him with his eyes and heard him with his ears."

Hassaan b. Thabit's poetry
Immediately after the Holy Prophet's speech, Hassan b. Thabit, the Companion and poet of the Messenger of Allah (S.A.W.), asked for his permission to compose a few verses of poetry about Imam 'Ali(A.S.) for the audience. The Holy Prophet (S.A.W.) said: "Say with the blessings of Allah". Hassan stood up and said: "O' people of Quraysh. I follow with my words what preceded and witnessed by the Messenger of Allah (S.A.W.). He then composed verses at the scene.

Hadith of Ghadir: Mutawatir
The following extracts (taken from authentic Sunni books) from the said lecture (khutbah) of the Holy Prophet are very important:
I am leaving behind, among you, two most precious things. . . (1) the Book of Allah . . . and (2) my descendants who are my family members. They will not separate from each other until they come to me near Kawthar (a pool in Paradise). Verily Allah is my Master and I am the Master of every believer. Then he took the hand of 'Ali and said:
"For whoever I am his Leader (mawla), 'Ali is his Leader (mawla)."
These two traditions are referred to as the traditions of 'Two Precious Things' (Thaqalayn) and Mastership' (Wilayah) . They are singly and jointly narrated by hundreds of traditionalists.

Asnad of Hadith of Ghadir:
a. al-Hafiz Abu 'Isa at-Tirmidhi (d.279 A.H.) has said in his Sahih (one of the as-Sihah as-Sittah) that "This is a good (hasan) and correct (sahih) hadith.''
b. al-Hafiz Abu Ja'far at-Tahawi (d. 321 A.H.) has said in his Mushkil u'l-athar that "This hadith is sahih according to the chains of narrators (asnad) and no one has said anything contrary to its narrators."
c. Abu 'Abdillah al-Hakim an-Naysaburi (d. 405 A.H.) has narrated this hadith from several chains in his al-Mustadrak and has said that this hadith is sahih.
d. Abu Muhammad Ahmad ibn Muhammad al-'Asim; has said: "This hadith is accepted by ummah, and it is in conformity with the principles.

The eternity of Ghadir event
The Wise God has willed the Ghadir historical event to remain in all ages and centuries, as a live history, attracting the hearts and minds, and the Islamic writers to discuss it in the commentary, history, tradition and theology books, and the religious orators to deliver lectures about it, and call it one of the undeniable virtues of Imam Ali (A.S.).
Not only the orators and lecturers, but also the poets too have been suggested by this event, and have lighted their literary talent through thinking and reflection on it, as well as increasing their loyalty to the owner of guardianship. They have left the best poems in different forms and various languages. Allamah Amini has mentioned an important part of Ghadir poems in every century of the Islamic history, while describing the attributes of the poets in the eleven volumes of Al-Ghadir book narrated from popular Islamic resources.
In other words, no other historical event in the world has been noted so much by different classes, including traditionist, commentator, theologian, philosopher, orator, poet, historian and biographer.
One of the reasons for eternity of this tradition is revelation of two Quranic verses in respect to this event, and since Quran is eternal and everlasting, this historical event too will be never forgotten.
110 people of tradition are narrators of this event
For showing importance of this historical event, it is enough to say that one hundred ten of the Prophet’s (S.A.W.) companions have narrated it.