Musnad Ahmad 26:51

Abu Bakr ibn Ayyash narrated to us, Asim narrated to me, from Zirr, from Ibn Mas’ud, who said: I was herding sheep for Uqbah ibn Abi Mu’ayt. The Messenger of Allah (peace and blessings of Allah be upon him) and Abu Bakr passed by me and said: "O boy, is there any milk?" I said: Yes, but I am a trustee. He said: "Is there any ewe that the ram has not mounted?" I brought him a ewe, and he wiped its udder. Milk descended, and he milked it into a vessel. He drank, and gave Abu Bakr to drink. Then he said to the udder: "Shrink," and it shrank. He said: Then I came to him after this and said: O Messenger of Allah, teach me some of these words. He wiped my head and said: "May Allah have mercy on you, for you are a taught boy."