Musnad Ahmad 26:757

Husayn ibn Ali narrated to us, from Za'idah, from Simak, from al-Qasim ibn Abd al-Rahman, from his father, from Abdullah, who said: We traveled by night with the Prophet, may Allah bless him and grant him peace. We said: "O Messenger of Allah, if only we could touch the ground and sleep, and let our mounts graze?" He said: "Do so." He said: Then he said: "Let some of you keep watch for us." Abdullah said: I said: "I will keep watch for you." He said: Then sleep overcame me and I slept. I did not wake up until the sun had risen, and the Messenger of Allah, may Allah bless him and grant him peace, did not wake up except by our talking. He ordered Bilal, so he gave the adhan, then he performed the iqamah for the prayer, and the Messenger of Allah, may Allah bless him and grant him peace, led us in prayer.