Musnad Ahmad 26:110

Yahya narrated to us, Shu'bah narrated to us, Jami' ibn Shaddad narrated to me, from 'Abd al-Rahman ibn Abi 'Alqamah, who said: I heard Ibn Mas'ud saying: The Prophet, peace and blessings of Allah be upon him, arrived from al-Hudaybiyah at night, and we descended upon a level stretch of land. He said: "Who will keep watch for us?" Bilal said: "I will." He said: "Then you will sleep." He said: "No." So he slept until the sun rose. Then so-and-so and so-and-so woke up, among them was 'Umar. He said: "Hasten." Then the Prophet, peace and blessings of Allah be upon him, woke up and said: "Do what you were accustomed to doing." When they had done so, he said: "Do it like this for whoever among you sleeps or forgets."