Maliks Muwatta Book 56, Hadith Number 24.

Malik related to me from Ishaq ibn Abdullah ibn Abi Talha that Anas ibn Malik said, “I heard Umar ibn al-Khattab, when I was going out with him to visit an orchard, say to himself, and there was a wall between him and me and he was inside the garden, ‘Umar ibn al-Khattab, amir al-muminin! Well done! Well done! By Allah, fear Allah or he will punish you.'”