Upon this...
Upon this, he sent for Imam Ali and asked him to prepare food consisting of goat meat and milk and invite all the children of ABD AL-MOTALIB, so that he might talk to them and convey to them God's message. Ali did so; they numbered about forty persons, including the Prophet's uncles, ABU TALIB, HAMZA and ABBAS…. When those persons had finished eating, the Prophet wanted to speak to them; but ABU LAHAB forestalled him, and the assembly dispersed without hearing Muhammad.
The Prophet (P) asked Imam Ali to do the same thing on the following day. When they had finished eating, Muhammad spoke to them thus; "In the whole of Arabia, I do not know anyone who has brought better things to his nation than what I have brought to you, which is good both for this world and the next. Verily God has commanded me to incite you to Him.
Who is there among you ready and willing to assist me and share my burden in this mission of mine, and be my vizier, my colleague, my WASI, and my KHALIFA". All of them remained silent. Then Ali, who was the youngest of all, stood up and said, "Oh Prophet of God, I am prepared to be your vizier, to share your burden with you, and be your KHALIFA". Upon this the Prophet (P) caught hold of the neck of Imam Ali and said, "Oh people, this Ali is my colleague, my WASI, and my KHALIFA.
You are all to hearken unto him and obey him". All of them got up laughing, and turning to ABU TALIB, they said, "Listen to thy son and obey him!". This is a very important matter and must be given due weight. The unequivocal term KHALIFA should be notices. After that solemn declaration of nomination, coming as it did in the form of an offer being accepted, did anything unusual happen to cancel it?
Its cancellation without good cause would have been very damaging to the reputation of a Prophet who claimed God's inspiration for his words; for of what use was that inspiration if it failed him on this momentous occasion and in this very important matter, arranged as it was under God's command?
From Muhammad's circumspective nature, it is evident that he would not have made this declaration in respect of a boy in his teens, especially in the presence of the elders of the family, many of whom were in the habit of deriding him and his uncles, among whom was the father of Imam Ali himself, unless he had been driven to do so God's command.