The very first person to embrace Islam was Hazrat Khadijah (radhiyallahu ‘anha). She embraced Islam on the very same day Rasulullah (sallallahu ‘alaihi wasallam) received prophethood. (Isaabah 8/99-100, Sharhuz Zurqaani 1/444)
According to some Ulama, the second person to embrace Islam was Waraqah bin Nawfal. He embraced Islam when Hazrat Khadijah (radhiyallahu ‘anha) took Rasulullah (sallallahu ‘alaihi wasallam) to speak to him and inform him of his encounter with Hazrat Jibreel (‘alaihis salaam) in the Cave of Hira. (Sharhuz Zurqaani 1/4451-453)
The third person to embrace Islam was Hazrat Ali (radhiyallahu ‘anhu). Hazrat Ali (radhiyallahu ‘anhu) accepted Islam the day after Rasulullah (sallallahu ‘alaihi wasallam) had received prophethood.
Rasulullah (sallallahu ‘alaihi wasallam) was blessed with prophethood on a Monday and Hazrat Ali (radhiyallahu ‘anhu) accepted Islam on a Tuesday. (Usdul Ghaabah 3/283) At that time, Hazrat Ali (radhiyallahu ‘anhu) was ten years old and was living in the care of Rasulullah (sallallahu ‘alaihi wasallam). (Seerah Ibnu Hishaam 1/246, Mustadrak Haakim #6463 and Majma’uz Zawaa’id #13469)
Rasulullah (sallallahu ‘alaihi wasallam) Inviting Hazrat Ali (radhiyallahu ‘anhu) to Embrace Islam
On the Monday evening after Rasulullah (sallallahu ‘alaihi wasallam) was blessed with nbuwwah, he was performing salaah with Hazrat Khadijah (radhiyallahu ‘anha) when Hazrat Ali (radhiyallahu ‘anhu) saw them and asked, “What is this?”
Rasulullah (sallallahu ‘alaihi wasallam) replied, “This is the deen of Allah Ta‘ala. The prophets of Allah Ta‘ala brought this very same deen to the world. I am inviting you to Allah Ta‘ala; to worship Him alone and renounce (the idols of) Laat and Uzza.”
Hazrat Ali (radhiyallahu ‘anhu) replied, “This is something utterly new which I have never heard before. Until I do not mention this to my father, Abu Taalib, I cannot make any decision.”
Since Rasulullah (sallallahu ‘alaihi wasallam) was reluctant for his secret to be revealed to anyone, he said, “O Ali! If you do not wish to embrace Islam, don’t mention it to anyone else.”
Not even a night had passed when the desire to accept Islam entered the heart of Hazrat Ali (radhiyallahu ‘anhu). The next morning, he presented himself before Rasulullah (sallallahu ‘alaihi wasallam) saying, “What are you inviting towards?” Rasulullah (sallallahu ‘alaihi wasallam) replied, “Bear testimony that Allah Ta‘ala is the only deity worthy of worship. Renounce Laat and Uzza and express your aversion and disassociation from idolatry.”
Hazrat Ali (radhiyallahu ‘anhu) then embraced Islam. For quite some time thereafter (some narrations mention that it was for one year), he concealed his Islam from his father, Abu Taalib. (Usdul Ghaabah 3/283, Seerah Halabiyyah 1/383, Seeratul Mustafa 1/155)