It was narrated from 'Ali bin Abu Talib that:
The Messenger of Allah said: "The eye is the leather strap (that ties up) the anus, so whoever falls asleep, let him perform ablution." (JA)
English reference (JA): Vol. 1, Book 1, Hadith 477 (JA)
Arabic reference (JA): Book 1, Hadith 515 (JA)