Musnad Ahmad 5:129

Husayn ibn Ali narrated to us, from Za'idah, from Simak, from Hanash, from Ali, who said: The Prophet, peace and blessings of Allah be upon him, said to me: "If two disputants come before you, do not hear the speech of the first until you hear the speech of the other; for then you will see how you judge." He said: Ali then said: "I have never ceased to be a judge after that."