Musnad Ahmed: Hadith 3633

The Prophet (ﷺ) came from al-Hudaibiyah at night, and we halted at some sandy ground. He said: “Who will keep watch?" Bilal said: I will. He said: “But you might fall asleep.” He said: No (I will not). But he slept until the sun rose, then some people woke up, including ‘Umar, who said: Speak. The Prophet (ﷺ) woke up and said: "Do what you usually do." When they had done that, he said: “This is what you should do, if anyone among you falls asleep or forgets.”