Musnad Abu Ya'la al-Mawsili 3:111

Zakariya ibn Yahya narrated to us, Sharik narrated to us, from Simak, from Hanash, from Ali, who said: The Messenger of Allah (ﷺ) sent me to a people of advanced age while I was young in age. He said: "If two disputing parties come to you, do not listen to one of them until you have heard from the other, for it will make the judgment clear to you." He said: So I learned [this], and I have never ceased to be a judge.