Musnad Abu Ya'la al-Mawsili 152:28

Ibrahim ibn al-Hajjaj al-Sami narrated to us, Salam ibn Sulayman Abu al-Mundhir narrated to us, Asim ibn Bahdalah narrated to us, from Zirr, from Abdullah ibn Mas’ud, who said: I was with the sheep of the family of Abu Mu’ayt, tending them, when the Prophet (peace be upon him) came to me, and with him was Abu Bakr ibn Abi Quhafah. The Prophet (peace be upon him) said: "O boy, do you have any milk to give us to drink?" I said: "Yes, but I am a trustee." He said: "Do you have a *shasum* ewe that the ram has not mounted?" I said: "Yes." So I brought him a *shasum* ewe—Salam said: "It is one that the ram has not mounted, and it is one that has no udder." The Prophet (peace be upon him) wiped the place of the udder, and it had no udder, and suddenly it was a full udder, filled with milk. I brought him a hollowed-out rock, and he milked into it, then gave Abu Bakr to drink, and gave me to drink, then he drank. Then he said to the udder: "Shrink," and it returned as it was. He said: I saw this with my own eyes from the Messenger of Allah (peace be upon him). I said: "O Messenger of Allah, teach me." He wiped my head and said: "May Allah bless you, for you are a taught boy." So I accepted Islam. I came to the Prophet (peace be upon him), and while we were with him at Hira, Surah al-Mursalat was revealed to him. I took it while his mouth was moist with it. I do not know with which of the two verses it ended: "And when it is said to them, 'Bow,' they do not bow" (al-Mursalat), or "Then in what statement after this will they believe?" (al-Mursalat). I took seventy surahs from the mouth of the Messenger of Allah (peace be upon him), and I took the rest of the Quran from his companions. He said: While we were sleeping at Hira, nothing awakened us except the saying of the Prophet (peace be upon him): "The one who prevented you from it has prevented it from you." We said: "O Messenger of Allah, what is that?" He said: "A snake that came out from the side of the mountain."