Sunan Ibn Majah Book 1, Hadith 211

The Messenger of Allah said: "The eye is the leather strap (that ties up) the anus, so whoever falls asleep, let him perform ablution."

Previous Hadith

"He would sleep like that while he was sitting up."

Sunan Ibn Majah Book 1, Hadith 210
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"The Messenger of Allah used to command us not to take off our leather socks for three days except in the case of sexual impurity, but not in the case of defecation, urine or sleep (i.e. during travel)."

Sunan Ibn Majah Book 1, Hadith 212