Sunan Ibn Majah, Chapter: The Chapters of Purification and its Sunnah

that the Messenger of Allah (saw) said: "The eye is the leather strap (that ties up) the anus,* so whoever falls asleep, let him perform ablution." (Daif) *What is meant is that when a person is asleep, it is as if the (muscle) straps tying his anus are loosened, because he may easily break wind in his sleep, thus invalidating his ablution without realizing it, so he should perform ablution when he wakes up.