Musannaf Ibn Abi Shayba 27:430

Abu Mu'awiyah narrated to us, from al-A'mash, from 'Amr ibn Murrah, from Abu al-Bakhtari, from 'Ali, who said: The Messenger of Allah (may Allah bless him and grant him peace) sent me to the people of Yemen to judge between them. I said: "O Messenger of Allah, I have no knowledge of judging." He struck my chest with his hand and said: "O Allah, guide his heart and make his tongue firm." I have not doubted in a judgment between two people since I sat in this position of mine.