First to Accept Islam from the Men

Hazrat Abu Bakr (radhiyallahu ‘anhu) was the first among the free men to accept the invitation of Rasulullah (sallallahu ‘alaihi wasallam) and enter into the fold of Islam. (Al-Bidaayah-wan-Nihaayah 3/28)

Spontaneously Accepting the Invitation of Rasulullah (sallallahu ‘alaihi wasallam)

Rasulullah (sallallahu ‘alaihi wasallam) mentioned:

ما دعوت أحدا إلى الإسلام إلا كانت له عنه كبوة وتردد ونظر إلا أبا بكر ما عتم حين ذكرته له ما تردد فيه

(At the beginning of Islam,) whoever I invited to Islam expressed some reservation, uncertainty and doubt regarding it, with the exception of Hazrat Abu Bakr (radhiyallahu ‘anhu). When I invited him to Islam, he readily embraced it, without an iota of doubt and reservation.

(Usdul Ghaabah 3/21)