Musannaf Ibn Abi Shayba 3:2359

Ghundar narrated to us, from Shu'bah, from Jami' ibn Shaddad, who said: I heard 'Abd al-Rahman ibn Abi 'Alqamah say: I heard 'Abdullah ibn Mas'ud say: We came with the Messenger of Allah (may Allah bless him and grant him peace) from al-Hudaybiyyah. They mentioned that they camped on *dahas* of the earth—meaning by *dahas*, sand. He said: The Messenger of Allah (may Allah bless him and grant him peace) said, "Who will keep watch for us?" Bilal said, "I will." The Prophet (may Allah bless him and grant him peace) said, "Then we shall sleep." He said: So they slept until the sun rose upon them. He said: Some people woke up, among them so-and-so and so-and-so, and among them was 'Umar. We said, "Speak up" (meaning: talk). He said: The Prophet (may Allah bless him and grant him peace) woke up and said, "Do as you used to do." He said: It is likewise for whoever sleeps or forgets.