(6) باب نواقض الوضوء
Narrated Mu'awiyah (rad):
Allah's Messenger (saW) said: "The eye (when awake) is the string of the anus (to stop air escaping), and if the two eyes sleep the string is untied". [Reported by Ahmad and At-Tabarani who added, "Whoever sleeps should perform ablution]"
Reference: Bulugh al-Maram 80
In-book reference: Book 1, Hadith 97
English translation: Book 1, Hadith 86