سنن ابن ماجه

It was narrated that 'Ali said: “The Messenger of Allah (ﷺ) sent me to Yemen. I said: 'O Messenger of Allah, you are sending me to judge between them while I am a young man, and I do not know how to judge.' He struck me on the chest with his hand and said: 'O Allah, guide his heart and make his tongue steadfast.' And after that I never doubted in passing judgment between two people.”