Musannaf Ibn Abi Shayba 36:61

Abdullah ibn Idris narrated to us, from Abu Malik al-Ashja'i, from Salim, who said: I said to Ibn al-Hanafiyyah, "Was Abu Bakr the first of the people in Islam?" He said, "No." I said, "In what did Abu Bakr excel and precede, such that no one is mentioned other than Abu Bakr?" He said, "He was the best of them in Islam from the time he accepted Islam until he joined his Lord."