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." (RU)
English reference (RU): Vol. 1, Book 1, Hadith 477 (RU)
Arabic reference (RU): Book 1, Hadith 515 (RU)