Musnad Abu Ya'la al-Mawsili 3:33

Zuhayr narrated to us, Ubayd Allah ibn Musa narrated to us, Shayban narrated to us, from Abu Ishaq, from Amr ibn Hubshi, from Ali, who said: The Messenger of Allah (may Allah bless him and grant him peace) sent me to Yemen. I said: "O Messenger of Allah, you are sending me to a people who are elders of advanced age, and I fear that I will not be correct [in my judgment]." He said: "Indeed, Allah will make your tongue firm and guide your heart."