Musnad Ahmad 5:75

Yahya narrated to me, from al-A’mash, from ‘Amr ibn Murrah, from Abu al-Bakhtari, from ‘Ali, who said: The Messenger of Allah, peace and blessings of Allah be upon him, sent me to Yemen while I was of young age. I said: "You are sending me to a people among whom there will be incidents, and I have no knowledge of judicial rulings?" He said: "Indeed, Allah will guide your tongue and make your heart firm." He said: So I never doubted a judgment between two people thereafter.