Almost all the commentators have written that the first believer in Islam was her holiness HAZRAT KHADIJEH the faithful wife to the messenger of God; and with no doubt the second convert was HAZRAT Imam Ali who was then a boy of ten or twelve years old. So HAZRAT Ali was the first Muslim and HAZRAT KHADIJEH the first muslimess.
HAKIM NISHABURI has written in his famous book; (MOSTADRAK MAARIFAT: PAGE 22):-there is no difference and disagreement among the historians, that HAZRAT Ali was the foremost and the first of man believers in Islam. Yet some commentators in opposite sects, have tried to reduce the importance of his conversion because of his low age; and also to by-pass the third conversion who was the prophet's adopted son (ZAID), and give the honour of being the first Muslim to ABUBAKR, their first adopted kalif, if he really was the fourth convert! And such endeavours are no more than political struggles.
The famous Sonni scholar, IBN-ABI-ALHADID who has a famous commentary on Nahjolbalagha has:ِ ``In several occasion HAZRAT Ali has made a mention that he was the first man who believed in the Messenger of God, and also he was the first man who prayed to God as a Muslim behind the prophet; while ABUBAKR has never claimed so in all his assertions. No one would ignore such a great advantage or by-pass it for some one else.''