Mishkat al-Masabih #295
Purification
The Arabic includes the chain of narration (sanad) and the collector's commentary; the English translation shows the Prophet's words only.
وَعَن مُعَاوِيَة بن أبي سُفْيَان أَنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: قَالَ: «إِنَّمَا الْعَيْنَانِ وِكَاءُ السَّهِ فَإِذَا نَامَتِ الْعَيْنُ اسْتطْلقَ الوكاء» . رَوَاهُ الدِّرَامِي
Mu'awiya b. Abu Sufyan reported the Prophet as saying, “The eyes are the leather strap of the anus, and when the eye sleeps the leather strap is loosened.”
Darimi transmitted it.
About Mishkat al-Masabih
This hadith is #295 from Mishkat al-Masabih, in the Book of Purification.