What is the evidence that sleep invalidates ablution?

Scholars have differed on several opinions regarding whether sleep invalidates ablution (wudu’). The preponderant view is that sleep invalidates ablution if it is deep and prolonged, such that the sleeper is unaware of anything occurring. It does not invalidate ablution if it is light and the sleeper does not lose awareness. They cite as evidence the hadith of Safwan ibn Assal concerning sleep invalidating ablution, and the hadith of Anas concerning it not invalidating it, as well as the Prophet's (peace be upon him) saying: “The eyes are the string of the anus; so when the eyes sleep, the string is loosened.”