Abu Bakr was the first free man to believe in Prophet, sallallaahu 'alayhi wa sallam
1-11-2003 | IslamWeb
Question:
What is the difference between a Muslim and a Mu'min? Who was the first person that believed in Prophet (Sallallahu Alaihi wa Sallam) Muhammad from the men is it: Abu Bakr or Ali or Zaid?
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad 
is His slave and Messenger.
With regard to the first question, please refer to Fatwa 83219.
As regards the second question, the scholars 
differed in opinion concerning the first person who embraced Islam at the hands of the Prophet, Muhammad, . Some of them are of the view that 'Ali 
was the first person who embraced Islam. This is based on the opinion of Ibn 'Abbaas which was narrated by Ibn Sa'd with a chain of narrators which Shaykh Al-Albaani 
graded as Hassan [Good] However, this chain reads that 'Ali embraced Islam after Khadeejah 
.
However, Zayd Ibn Arqam said that 'Ali was the first person who embraced Islam. [At-Tirmithi]
Some others said that Abu Bakr was the first person who embraced Islam. This was said by Ibraheem An-Nakha'i and reported by At-Tirmithi who graded it as Saheeh [authentic]
Imaam Ibn Katheer said: "This is the opinion of the majority of the scholars of Ahlus-Sunnah."
Also, some scholars said that the first person who embraced Islam was Zayd Ibn Haarithah ; this is said by Az-Zuhri as reported by 'Abdur-Razaaq in his Musnaf.
It is also said that the first person who embraced Islam was Khadeejah ; this is the view of Ibn 'Abbaas and Ibn Ishaaq 
.
However, there are many authentic narrations that prove that the first person ever who embraced Islam was Khadeejah since the Prophet came back to her, right after having encountered the angel Jibreel (Gabriel) 
in Ghaar Hiraa', and she comforted him and said, "O no, I swear, By Allaah, He will never humiliate you…" [Al-Bukhari ]
Ibn Katheer said, "To reconcile between all these narrations, Khadeejah was the first person who embraced Islam among women, and the narrations mean that she even preceded men; Zayd was the first slave who embraced Islam; 'Ali was the first child who embraced Islam and Abu Bakr was the first man who embraced Islam, and his acceptance of Islam was more beneficial than those mentioned above."
Allaah Knows best.