Atiyyah bin Qays narrates: I heard Sayyiduna Mu'awiyah bin Abi Sufyan (may Allah be pleased with him) say: I heard the Messenger of Allah (peace and blessings be upon him) say: "The eye is the drawstring of the backside; when the eye sleeps, the drawstring comes loose. So whoever sleeps, let him perform ablution (wudu)."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16924