He accepted Islam

‘Aisha Radiyallahu ‘anha narrates that: Abu Bakr Radiyallahu ‘anhu came out to see Rasulullah Sallallahu ‘alaihi wasallam, who was his friend during the period of ignorance. On seeing him he said: O Abul Qasim! You are not seen in the gathering of your people, and they accuse you of finding faults in their forefathers. Rasulullah Sallallahu ‘alaihi wasallam said: “Verily, I am Allah’s Messenger, I invite you to Allah.” As soon as Rasulullah Sallallahu ‘alaihi wasallam completed his words Abu Bakr Radiyallahu ‘anhu embraced Islam. When Rasulullah Sallallahu ‘alaihi wasallam returned from him, there was no one between the two mountains of Makkah who was happier than him on the acceptance of Islam by Abu Bakr Radiyallahu ‘anhu. (Bidayah-wan-Nihayah)

Note: ‘Qasim, is the name of the son of Rasulullah Sallallahu ‘alaihi wasallam; and ‘Abul Qasim is his kuniyyah, the title usually given to any person with the name of his son or his parent.