Musnad Ahmad 26:870

Muhammad ibn Ja’far narrated to us, Shu’bah narrated to us, from Jami’ ibn Shaddad, who said: I heard ‘Abd al-Rahman ibn Abi ‘Alqamah say: I heard ‘Abdullah ibn Mas’ud say: We were returning with the Messenger of Allah, peace and blessings of Allah be upon him, from al-Hudaybiyyah. They mentioned that they camped on *dahas* of the earth—meaning by *dahas*: sand. He said: "Who will keep watch for us?" Bilal said: "I will." The Messenger of Allah, peace and blessings of Allah be upon him, said: "Then you will sleep." He said: So they slept until the sun rose. Some people woke up, among them so-and-so and so-and-so, including ‘Umar. He said: We said: "Speak up"—meaning talk. He said: The Prophet, peace and blessings of Allah be upon him, woke up and said: "Do as you were doing." He said: So we did. He said: And he said: "Do likewise for whoever sleeps or forgets." He said: The she-camel of the Messenger of Allah, peace and blessings of Allah be upon him, went astray, so I searched for it and found its rope caught on a tree. I brought it to the Prophet, peace and blessings of Allah be upon him, and he rode it, pleased. When revelation would descend upon the Prophet, peace and blessings of Allah be upon him, it would be intense for him, and we would recognize that in him. He said: He moved away, withdrawing behind us. He said: He began to cover his head with his garment, and it became intense for him, until we knew that it had been revealed to him. He came to us and informed us that it had been revealed to him: "Indeed, We have given you a clear conquest" [Al-Fath: 1].