In “The History of Islam”, vol. 1, by Akbar Shah Najeebabadi and revised by Safi-ur-Rahman Mubarakpuri, it states on page 106:”He started the task of keeping people away from polytheism and inviting to Tauhid right from his household, with the result that Khadijah radiAllaahu ‘anha was the first to embrace Islam. ‘Ali bin abu Talib radiAllaahu ‘anhu and Zaid bin Harithah radiAllaahu ‘anhu were also converted to Islam the very first day. These all belonged to his household. Abu Bakr bin abu Quhafah radiAllaahu ‘anhu who was the Prophet’s friend, accepted Islam the same day. Now, the individuals to become Muslims on the first day were his family members and his best friend. These were the people who knew the Prophet salAllaahu ‘alayhi wa salam well and were aware of his traits and character for a very long time, and no aspect of his life was a secret to them. Now, their embracing of Islam the very first day was an outstanding proof of his truthfulness and uprightness.”
The History of al-Tabari, volume VI, Muhammad at Mecca, cites a lot of reports with their chains of transmissions on this subject of who became the first Muslim after Khadijah; there are reports that ‘Ali is the first, there are reports that abu Bakr is the first and there are reports that Zaid is the first radiAllaahu ‘anhum.The pages where these reports are listed are pages 80-86.