Ghadir and the Prophet's Successor
Ghadir is the day in which The Holy Prophet Muhammad P.B.U.H&H.F declared the succession of imam Amir Al Momenin Ali Ibn Abitalib A.S
It was in 10 A.H. and the time for Hajj. The Hijazi deserts witnessed large crowds of Muslims who unitedly chanted the same slogans and proceeded towards the same holy end.
That year the sight of tie Hajj pilgrimage was much more exciting and moving than ever before. Muslims most hurriedly and eagerly traversed the way and went to Makkah - this holy city.
The celestial melody of 'Labayka', 'Yes, I have come', resounded through Makkah. Caravans reached the city one after the other. The hajjis unitedly and harmoniously in pilgrim's garb, while shedding tears of joy and love for God, hurried to the sacred threshold of God and circumambulated the Ka'aba - the Holy House built by the champion of monotheism - Abraham, the Friend of God.
Farid Vajdi has calculated the number of hajjis to have been 90,000  in the year 10 A.H., but there are some who hold that
the number was 124,000." 
The Holy Prophet of Islam watched that splendid scene with the utmost affection and eagerness. He was pleased to observe that the Masjid al-Haram was overflowing with Muslims who had gathered together in conformity with the holy precept, 'Truly the faithful are brothers', and were worshipping God like brothers and angels.
The Holy Prophet was clearly happy with his great achievement - with having fulfilled his divine mission in the best manner possible.
Nevertheless, his resplendent face was sometimes covered with a halo of sorrow and anxiety, and his pure heart filled with sadness and worry.
He was in fact worried about the fate of the Muslims after his leaving this world for heaven. He feared lest after him the society of Muslims should break apart; Muslims should disperse, the spirit of unity and fraternity should vanish among them, and consequently they regress.
Obviously, the Holy Prophet of Islam was well aware of the fact that the Ummah of Islam was in great need of honest, knowledgeable leaders, or otherwise the fruits of his years of efforts would all be wasted.
For this reason, whenever he was going to leave Medina either for war or for other purposes, even if his trip was short, he would assign a competent, trustworthy person to supervise their affairs and would never leave the people of Medina without any guardian and supervisor. 
Thus, how is it possible to imagine that such a compassionate, sympathetic prophet might have left the momentous affairs of his beloved Ummah of Islam to chance and not have designated any reliable administrator for them.
And no doubt he knew very well who deserved the position of the caliphate of the Muslims and for whose mature stature the garb of the caliphate had been sewn.
That celebrated man was the same who, in the presence of the chiefs of the Quraysh and the relatives of the Prophet who had been invited to the House of the Prophet of Islam at the beginning of his prophetic mission, had been acknowledged as the successor of the Holy Prophet of Islam by the Prophet himself. 
He was a pious, God-fearing man who did not associate anything with God and did not prostrate before idols even for a single moment.
He was a sacrificial soldier of Islam. His knowledge originated in the knowledge of the Prophet of God and his judgment was the best. 
He was well-known. He was 'Ali, son of Abu Talib.
The Hajj ceremonies were over, and the Muslims were preparing to move towards their own towns when suddenly the call of the herald of the Holy Prophet of Islam resounded in the Hijaz desert and made the Muslims stop. His heralds called on the people to gather together again.
The Muslims, of course, did not know why they had been given this command, but the fact was that the Angel of Revelation had descended and conveyed this verse to the Prophet, 'O Prophet! Deliver what has been revealed to you from your Lord, and if you do it not, then you have not delivered His message, and God will protect you- from the people; surely God will not guide the unbelieving people' (5:67).
The issue about which God spoke to His Prophet in such a serious tone was nothing other than the formal announcement of the caliphate of 'Ali, the significant subject that the Prophet hesitated to declare, for he feared lest this announcement should cause dissension and discord among the Muslims and was thus waiting for a favourable occasion to make clear the matter to them.
Upon receiving this revelation, he knew that the time had come for the crucial purpose. So he, immediately assembled the Muslims at Ghadir Khum, which was a hot, arid desert, to clarify the vital issue in Islam - the issue of the caliphate.
The people started wondering why that command had been issued, but before long the congregational ritual prayers was announced and after saying the noon ritual prayers, the crowd of Muslims witnessed the celestial, enchanting countenance of the Prophet over a pulpit made of saddles of camels.
A profound silence prevailed. Then the divine, meaningful words of the Prophet broke the silence of the Hijaz desert. After praising Almighty God, he announced the heart-rending news of his oncoming death and then asked the Muslims, 'O people! What kind of a prophet have I been for you?'
All exclaimed unitedly, 'O Prophet of God! You did your best to admonish and rectify us and never neglected to train us and led us onto the path of piety. May God reward you best'.
The Holy Prophet of Islam said, 'After me, God's Book and the sinless leaders are side by side your leader and guide. You should perfectly follow them, so you will not go astray'.
Then he took 'Ali by the hand, lifted him so that all would see him and exclaimed, 'O people! Who is the guardian and supervisor of the faithful?'
The Muslims answered, 'God and His Prophet know best'.
The Holy Prophet of Islam said, 'God is my Master and I am the Master of the faithful'. Then he added without any pause, "Ali is the Master of those whose Master I am. Almighty God be the friend of his friend and be the foe of his foe. Help those who help him and frustrate the hope of those who betray him...'.
The Prophet repeated the sentence, "Ali is the Master of those whose Master I am', three times. At the end of the speech he said, 'Those present should convey this truth to those who are absent'.
The crowd of Muslims had hardly dispersed when this verse was revealed to the Prophet, 'This day have I perfected for you your religion and completed My favour on you and chosen for you Islam as a religion' (5:3).
After the magnificent ceremonies of designating the successor of the Holy Prophet of Islam were over, the Muslims hurried to congratulate 'Ali for being appointed as the Prophet's successor and Caliph.
Abu Bakr was the first to congratulate 'Ali and 'Umar was the second. They parted with 'Ali while saying the following words, 'Blessed are you, son of Abu Talib, who have become my Master and every believer's Master'. 
THE NARRATORS OF GHADIR
As a matter of fact, there are more than 120,000 narrators of Ghadir. According to the command of the Prophet, the Muslims present at Ghadir regarded the incident of Ghadir and the issue of appointing 'Ali as the successor of the Prophet as most significant and narrated it to the others.  And it was for this reason that in public gatherings of Muslims, the reminiscence of Ghadir was renewed repeatedly.
About 25 years after the day of Ghadir, when most of the faithful companions and followers of the Holy Prophet of Islam had passed away, and just a few were still alive, 'Ali asked the people to bear witness if they had been present in Ghadir and heard the Ghadir tradition from the blessed mouth of the Prophet. Immediately 30 people stood up and narrated the Ghadir tradition. 
In 58 or 59 A. H., a year before the death of Mu'awiyah, Imam Husayn, peace be upon him, assembled the Bani Hashim and Ansar and other hajjis at Mana and, during an extremely moving speech, asked them, 'I swear to you by God to speak out if you know that on the day of Ghadir, God's Prophet appointed 'Ali as the Master and Leader of the Ummah of Islam and commanded the audience to convey this message to the others'. All said that they knew this fact. 
Sunni scholars have mentioned in their reliable books the names of 110 companions of the Prophet who had heard this tradition from the Holy Prophet of Islam and had narrated it to others.  Even a number of scholars and Islamic theologians wrote special books on Ghadir. 
THE PURPORT OF THE DISCOURSE ON GHADIR
The available documents reveal that the words mawla (master) and vali (guardian) refer to the successor of the Holy Prophet of Islam and the Guardian of the Ummah of Islam, and that no other meaning can be applied to these two words.
Now, take notice of the following points:
We have realized that the Holy Prophet of Islam was hesitant to propound the Ghadir tradition and that he did not declare it until God openly and seriously commanded him to do so.
It is totally wrong to hold that by the Ghadir tradition the Prophet meant to remind the people of the position of 'Ali as a friend of the Holy Prophet of Islam and the Muslims. If that were the case, the Holy Prophet of Islam would never have hesitated to announce it, for obviously such an announcement would cause no discord or dissension among the Muslims.
Thus the Holy Prophet of Islam surely had reference to the issue of the caliphate and the assignment of his own successor, which was clearly likely to elicit the mutiny and mischief of ambitious opportunists.
Before uttering the well-known sentence, "Ali is the Master of those whose Master I am', the Prophet asked the audience to admit that he himself was their guardian and leader and that he was to be obeyed by them, and after the people present in Ghadir Khum had admitted this fact, the Holy Prophet of Islam attributed the same position to 'Ali immediately, saying "Ali is the Master and Leader of anybody whose Master and Leader I am'.
With the permission of the Prophet, Hissan ibn Sabit composed a poem about Ghadir Khum and circulated it. In this poem, the position of the Caliphate and Imamate of 'Ali have been openly expressed and specified. No one among that great crowd of Muslims protested that Hissan had misapplied the word mawla (master). Rather, Hissan was confirmed and applauded for this poem.
The poem, in effect, said, 'After the Holy Prophet of Islam had the people admit that he was their divine Master and religious leader, he said to 'Ali, "Stand up, 'Ali. I assent to your Leadership and Imamate after myself. Then, "Ali is the Master and Leader of anybody whose Master and Leader I am. You should all be loyal followers and sincere friends of 'Ali'". 
After the Ghadir ceremonies were over, the Prophet, together with 'Ali, sat in a tent and ordered all the Muslims, even the women of his own family, to congratulate 'Ali, to swear allegiance to him, and to greet him as the Commander of the Faithful.  It is obvious that all these ceremonies and orders testify to nothing other than the designation of 'Ali as the Caliph and Imam of the Muslims by the Holy Prophet of Islam.
Twice the Prophet said to the people, 'Congratulate me, for God specifically appointed me Prophet and my family imams'. 
These testimonies and documents leave no doubt about the Ghadir Khum tradition and the caliphate of 'Ali.
. Encyclopedia of Farid Vadji, Vol. 3, p.542.
. Al-Ghadir, Vol. 1, p.9.
. Kamil, pol.p.216, 278, 242.
. Tarikhi Tahari, Vol. 3, pp.1171-1173.
. Fadail ul-Khamsih, printed by Dar ul-Khutub ul-Islamiyah, Vol. 1, pp.178-186.
. Al-Ghadir, Vol. 1, pp.9-I1.
. Ibid., pp.60-61.
. Al-Ghadir, Vol. 1, pp.166-174.
. Ibid., pp.198-199.
. Ibid., pp.14-61.
. Twenty-six have been mentioned in the first volume of Al-Ghadir, pp.152-157.
. Al-Ghadir, Vol. 2, pp.34-41.
. Ibid., Vol. 1, pp.270-271.
. Ibid., pp.274.