Musannaf Ibn Abi Shayba 35:48

Ghundar narrated to us, from Shu'bah, from Jami' ibn Shaddad, who said: I heard 'Abd al-Rahman ibn Abi 'Alqamah, who said: I heard 'Abdullah ibn Mas'ud, who said: We were coming with the Prophet, may Allah bless him and grant him peace, from al-Hudaybiyyah. They mentioned that they descended upon *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. He said: Some people woke up, among them so-and-so and so-and-so, and among them was 'Umar ibn al-Khattab. He said: We said: "Speak loudly"—meaning, talk. He said: The Prophet, may Allah bless him and grant him peace, woke up and said: "Do as you were doing." He said: So we did. He said: He then said: "It is likewise for whoever sleeps or forgets."