Musnad Ahmad 5:319

Aswad ibn Amir narrated to us, Sharik narrated to us, from Simak, from Hanash, from Ali, who said: The Messenger of Allah (peace and blessings of Allah be upon him) sent me to Yemen. I said: "O Messenger of Allah, you are sending me to a people older than me, and I am a youth who does not understand how to judge?" He said: He placed his hand on my chest and said: "O Allah, make his tongue firm and guide his heart. O Ali, when two disputants sit before you, do not judge between them until you hear from the other just as you heard from the first; for if you do that, the judgment will become clear to you." He said: So no judgment ever differed for me after that, or no judgment ever became obscure to me after that.