Musnad Ahmad 5:644

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 then you will see how you judge." He said: So I have never ceased to be a judge after that.