Musnad Abu Ya'la al-Mawsili 3:141

Ubayd Allah ibn Umar narrated to us, Yahya ibn Sa'id narrated to us, from al-A'mash, from Amr ibn Murrah, from Abu al-Bakhtari, from Ali, who said: The Messenger of Allah (may Allah bless him and grant him peace) sent me to Yemen while I was young in age, having no knowledge of judicial rulings. He struck my chest and said: "Indeed, Allah will guide your heart and make your tongue firm." He said: So I never doubted a judicial ruling between two people after that.