From ‘Ali ibn Abi Talib (may Allah be pleased with him), who said: The Messenger of Allah (SAW) said: The eyes are the strap of the anal sphincter, so whoever falls asleep should perform ablution.
- Sunan Abi Dawud, The book of purity

His word “the strap of the anal sphincter [sah]” is the subject, and “the eyes” are its predicate. The sah refers to the hole of the rear end; it is part of the anus. … The meaning of this hadith is that a person, while he is awake, his anus is essentially fastened as if it were tied up with a strap, but whenever he falls asleep, its strap is loosened. This figure of speech is used to allude to bodily occurrences [hadath] and the passing of gas, and is among the best and most pleasant allusions, in which wakefulness is ascribed to the anus like a strap to a waterskin; just as the strap prevents what is inside the waterskin from coming out, likewise wakefulness prevents the anus from having an occurrence except if you force it. And the eye is used as an allusion to wakefulness, since someone who is asleep has no eyes to perceive.