Musnad Ahmad 31:773

Husayn ibn Muhammad narrated to us, Fitr narrated to us, from Isma'il ibn Raja' al-Zubaydi, from his father, who said: I heard Abu Sa'id al-Khudri saying: We were sitting waiting for the Messenger of Allah, may Allah bless him and grant him peace, and he came out to us from one of the houses of his wives. He said: So we stood up with him, and his sandal strap broke, so 'Ali stayed behind to mend it. The Messenger of Allah, may Allah bless him and grant him peace, proceeded, and we proceeded with him. Then he stopped to wait for him, and we stopped with him. He said: "Indeed, among you is one who will fight for the interpretation of this Qur'an, just as I fought for its revelation." We looked up, and among us were Abu Bakr and 'Umar. He said: "No, but it is the mender of the sandal." He said: So we went to give him the good news, and it was as if he had already heard it.