Musnad Ahmad 5:580

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from 'Amr ibn Murrah, he said: I heard Abu al-Bakhtari al-Ta'i, he said: One who heard 'Ali informed me, saying: When the Messenger of Allah, peace and blessings of Allah be upon him, sent me to Yemen, I said: "You are sending me while I am a man of young age, and I do not have knowledge of much of the judiciary?" He said: The Messenger of Allah, peace and blessings of Allah be upon him, struck my chest and said: "Go, for Allah, the Almighty and Majestic, will make your tongue firm and guide your heart." He said: "So, no judgment between two people has ever perplexed me."