Musnad Ahmad 26:163

Yazid narrated to us, Mas'udi informed us, from Jami' ibn Shaddad, from Abd al-Rahman ibn Abi Alqamah al-Thaqafi, from Abdullah ibn Mas'ud, who said: When we returned from the expedition of al-Hudaybiyyah, the Messenger of Allah (peace and blessings of Allah be upon him) said: "Who will guard us tonight?" Abdullah said: I said: "I." He said: "You will sleep." Then he repeated: "Who will guard us tonight?" I said: "I." He continued until he repeated it several times, and I said: "I, O Messenger of Allah." He said: "Then you." He said: So I guarded them. When it was the break of dawn, the words of the Messenger of Allah (peace and blessings of Allah be upon him), "You will sleep," overcame me, and I slept. Nothing woke us except the heat of the sun on our backs. The Messenger of Allah (peace and blessings of Allah be upon him) stood up and performed as he used to perform of ablution and the two rak'ahs of the dawn prayer, then he led us in the morning prayer. When he finished, he said: "If Allah, the Mighty and Majestic, had willed that you not sleep through it, He would not have let you sleep, but He willed that you be an example for those after you; so this is for whoever sleeps or forgets." He said: Then the she-camel of the Messenger of Allah (peace and blessings of Allah be upon him) and the camels of the people scattered. The people went out in search of them and brought back their camels, except for the she-camel of the Messenger of Allah (peace and blessings of Allah be upon him). Abdullah said: The Messenger of Allah (peace and blessings of Allah be upon him) said to me: "Go this way." I went where he told me, and I found its rein entangled in a tree, which could not have been untied except by a hand. He said: So I brought it to the Prophet (peace and blessings of Allah be upon him) and said: "O Messenger of Allah, by the One who sent you with the truth as a Prophet, I found its rein entangled in a tree, which could not have been untied except by a hand." He said: And Surah al-Fath was revealed to the Messenger of Allah (peace and blessings of Allah be upon him): "Indeed, We have given you a clear conquest." [Al-Fath: 1]