Musnad Ahmad 31:299

Waki’ narrated to us, Fatar narrated to us, from Isma’il ibn Raja’, from his father, from Abu Sa’id, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, among you is one who will fight for its interpretation, just as I fought for its revelation." He said: So Abu Bakr stood up, and ‘Umar [stood up], but he said: "No, but it is the one who is mending the sandal," and ‘Ali was mending his sandal.