Abu Dawud Book 2, Hadith Number 447.

Narated By Abdullah ibn Mas’ud : We proceeded with the Apostle of Allah (PBUH) on the occasion of al-Hudaybiyyah. The Apostle of Allah (PBUH) said: Who will keep watch for us? Bilal said: I (shall do). The overslept till the sun arose. The Prophet (PBUH) awoke and said: Do as you used to do (i.e. offer prayer as usual). Then we did accordingly. He said: Anyone who oversleeps or forgets (prayer) should do similarly.