Musnad Abu Ya'la al-Mawsili 151:19

Al-Mu'alla ibn Mahdi narrated to us, Abu 'Awanah narrated to us, from 'Asim ibn Bahdalah, from Zirr, from 'Abdullah ibn Mas'ud, who said: I was a young, growing boy tending the sheep of 'Uqbah ibn Abi Mu'ayt. The Prophet, peace and blessings of Allah be upon him, came along with Abu Bakr, and he said, "O boy, do you have any milk with you?" I said, "Yes, but I am a trustee (entrusted with them)." He said, "Bring me a sheep that the ram has not mounted." I brought him a young female goat or a lamb. The Messenger of Allah, peace and blessings of Allah be upon him, held it, then began to wipe the udder and supplicate until it let down milk. Abu Bakr brought him a rock, and he milked into it. Then he said to Abu Bakr, "Drink." So Abu Bakr drank, then the Prophet, peace and blessings of Allah be upon him, drank after him. Then he said to the udder, "Shrink," and it shrank, returning to how it was. He said: Then I came to the Prophet, peace and blessings of Allah be upon him, afterwards and said, "O Messenger of Allah, teach me from this speech or from this Quran." He wiped my head and said, "Indeed, you are a taught boy." He said: So I took seventy surahs from his mouth, and no human has disputed me regarding them.