Musnad Ahmad 26:861

'Affan narrated to us, Hammad ibn Salamah narrated to us, from 'Asim ibn Bahdalah, from Zirr ibn Hubaysh, from Ibn Mas'ud, that he said: I was a young, adolescent boy tending sheep for 'Uqbah ibn Abi Mu'ayt. The Prophet, peace and blessings of Allah be upon him, and Abu Bakr, may Allah be pleased with him, came, having fled from the polytheists. They said: "O boy, do you have any milk to give us to drink?" I said: "I am a trustee, and I am not going to give you to drink." The Prophet, peace and blessings of Allah be upon him, said: "Do you have a young ewe that has not been mounted by a ram?" I said: "Yes." So I brought it to them. The Prophet, peace and blessings of Allah be upon him, restrained it, wiped the udder, and supplicated, and the udder filled. Then Abu Bakr, may Allah be pleased with him, brought him a hollowed rock, and he milked into it. He drank, and Abu Bakr drank, then I drank. Then he said to the udder: "Shrink," and it shrank. I came to him after that and said: "Teach me some of these words." He said: "You are a taught boy." He said: So I took seventy surahs from his mouth, and no one disputes with me regarding them.