سنن أبي داود

Abu Qatadah reported: "The Prophet (ﷺ) was on a journey. The Prophet (ﷺ) took a turn and I also took a turn with him. He said: 'Look!' I said: 'This is a rider; these are two riders; and these are three' until we became seven. He then said: Guard for us our prayer, i.e. the Fajr prayer. But sleep dominated them and none could awaken them except the heat of the sun. They stood up and drove away a little. Then they got down (from their mounts) and performed ablution. Bilal called for prayer and they offered two rak'ahs of (Sunnah) of Fajr and then offered the Fajr prayer and mounted (their mounts). Some of them said to others: We showed negligence in prayer. The Prophet (ﷺ) said: There is no negligence in sleep. The negligence is in wakefulness. If any of you forget saying prayer, he should offer it when he remembers it and next day (he should say it) at its proper time.