Musnad Abu Ya'la al-Mawsili 3:56

Ubayd Allah narrated to us, Ghundar narrated to us, Shu'bah narrated to us, from 'Amr, who said: I heard Abu al-Bakhtari say: He informed me that he heard 'Ali say: When the Messenger of Allah (may Allah bless him and grant him peace) sent me to Yemen, I said: "You are sending me, and I am a man of young age, and I do not have much knowledge of judicial matters." He said: So he struck my chest and said: "Go, for Allah will make your tongue firm and guide your heart." He said: So no judgment between two people has ever baffled me.