Musnad Ahmad 5:105

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