Bulugh al-Maram
Bulugh al-Maram 80
وَعَنْ مُعَاوِيَةَ - رضى الله عنه - قَالَ: قَالَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -{ الْعَيْنُ وِكَاءُ السَّهِ, فَإِذَا نَامَتْ اَلْعَيْنَانِ اِسْتَطْلَقَ اَلْوِكَاءُ } رَوَاهُ أَحْمَدُ, وَالطَّبَرَانِيُّ وَزَادَ { وَمَنْ نَامَ فَلْيَتَوَضَّأْ }
Unreviewed translation. All hadiths in this collection are available in five languages. Titles and introductions may remain in Arabic.
Mu‘awiyah (may Allah be pleased with him) reported that the Messenger of Allah (peace and blessings be upon him) said: ‘The eye is the tether of the anus; when the two eyes sleep, the tether is released.’ Ahmad and al-Tabarani reported it, and al-Tabarani added: ‘Whoever sleeps should perform ablution.’
Notes and open questions — unreviewed
The wording uses a metaphor: the eye is likened to a tether that restrains the anus; when a person sleeps, that restraint is released. The English witness supplies an explanatory gloss about air escaping.