One night...
One night, laylatul-qadr (the Night of Majesty) a voice addressing him, commanded "Recite in the Name of thy Lord." (Surah al-‘Alaq 96:1).
Deeply excited by the strange phenomena of Divine Visitation, Muhammad hurried home to his wife, Khadijah, who listened to him attentively and said that "I bear witness that you are the Apostle of God." After an interval, the voice from heaven spoke again "O thou shrouded in thy mantle, arise, and warn, and magnify thy Lord." (Surah al-Muddathir 74:1-3) This was a signal for him to start preaching the gospel of truth of One God.
In the beginning Muhammad invited only those near him, to accept the new Faith. The first to embrace Islam among women was Khadijah and among men `Ali. Soon after, Zayd ibn al-Harithah became a convert to the new Faith followed by Abu Bakr and `Uthman. `Umar hitherto a violent opponent of Islam, notorious for the persecution of the Muslims and a bitter enemy of the Prophet, embraced Islam, later.
Caliph For three long years, he laboured quietly to wean his people from the worship of idols and drew only thirty followers. Muhammad now decided to appeal publicly to the Quraysh to give up idol worship and embrace Islam. He invited forty of his kinsmen to a feast. At the gathering, Muhammad stated that he had lived a lifetime among them and asked if they had ever found him lying?
The general response was "We have never found you lying, O al-Amin!" The Prophet of Islam asked if he were to tell them that their enemies had collected beyond the saridhills to attack them, would they believe? The reply was, "Yes." "Will you believe anything I will now say?" Again the reply was, "Yes." The Prophet addressed them: “I know no man in all Arabia, who can offer his kindreds, a more excellent thing than I now do. I offer you happiness both in this life and that which is to come.
God Almighty has commanded me to call unto Him. Who, therefore, among you will assist me herein shall become my Brother and my Caliph?” All of them hesitatingly declined the matter. `Ali (who was then given the title of "Amiru'1-Mu'minin" (the Commander of the faithful) by the Prophet, stood up and declared that he would assist the Prophet and vehemently threatened those who would oppose him.
Muhammad with great demonstration of affection, embraced `Ali and declared to all, to hear and obey `Ali as his Deputy and Caliph. The gathering broke out into laughter, taunting Abu Talib that now he will have to obey his son.