Musnad Abu Ya'la al-Mawsili 152:243

On the authority of Abdullah, he said: I was a young, growing boy, and I used to herd sheep for Ibn Abi Mu'ayt. The Prophet, peace and blessings of Allah be upon him, and Abu Bakr came, having fled from the polytheists. They said: "O boy, do you have any milk to give us to drink?" I said: "Yes, but I am a trustee, and I will not give you to drink." The Prophet, peace and blessings of Allah be upon him, said: "Do you have a young ewe that the ram has not yet mounted?" I said: "Yes." I brought it to him, and the Messenger of Allah, peace and blessings of Allah be upon him, held it, wiped the udder, and supplicated. The milk began to flow. Abu Bakr brought him a hollowed-out rock, and he milked into it. He drank, Abu Bakr drank, and I drank. Then he said to the udder: "Shrink," and it shrank. I came to him after that and said: "O Messenger of Allah, teach me from this speech or from this Quran." He said: "You are a taught boy." But I said: "Teach me." So I took from his mouth seventy surahs, and no one disputes with me regarding them.