Musnad Ahmad 5:772

Yahya ibn Adam narrated to us, Isra'il narrated to us, from Abu Ishaq, from Harithah ibn Mudarrib, from Ali, may Allah be pleased with him, who said: The Messenger of Allah, peace and blessings of Allah be upon him, sent me to Yemen. I said: "You are sending me to a people who are older than me so that I may judge between them." He said: "Go, for Allah will guide your heart and make your tongue firm."