Musannaf Ibn Abi Shayba 20:56

Abu Bakr narrated to us, he said: Husayn ibn Ali narrated to us, from Za'idah, from Simak, from Hanash, from Ali, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If two men come to you for judgment, do not judge for the first until you hear what the other says, for you will then see how you judge." Ali said: "I have never ceased to be a judge after that."