Musannaf Ibn Abi Shayba 34:33

Waki’ narrated to us, from Shu’bah, from ‘Amr ibn Murrah, from Abu Hamzah, the freed slave of the Ansar, from Zayd ibn Arqam, who said: "The first to accept Islam with the Messenger of Allah, may Allah bless him and grant him peace, was ‘Ali." I mentioned this to Ibrahim, and he denied it and said: "Abu Bakr."