Musannaf Ibn Abi Shayba 29:150

Waki' narrated to us, from Shu'bah, from 'Amr ibn Murrah, from Abu Hamzah al-Ansari, 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 that to Ibrahim, and he denied that and said: "Abu Bakr."