Musannaf Ibn Abi Shayba 13:3038

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Sufyan, from Ja’far ibn Muhammad, from his father, that the Prophet (peace and blessings of Allah be upon him) judged by the testimony of one witness and an oath. He said: And Ali (may Allah be pleased with him) judged by it among you.