Musannaf Ibn Abi Shayba 27:443

Aswad ibn Amir narrated to us, from Sharik, from Mansur, from Rib'i, from Ali, from the Prophet (may Allah bless him and grant him peace), who said: "O assembly of Quraysh, Allah will surely send against you a man from among you whose heart Allah has tested for faith, and he will strike you, or he will strike your necks." Abu Bakr said, "Am I he, O Messenger of Allah?" He said, "No." Then Umar said, "Am I he, O Messenger of Allah?" He said, "No, but he is the one mending the sandal." And he had given Ali his sandal to mend.