Musannaf Ibn Abi Shayba 13:3193

Abu Bakr narrated to us, he said: Husayn bin 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 hear what the first one says 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 since then."