Abu Huraira related that the Prophet (SAW) said: Whenever any of you awakens from his sleep, he should not put his hand into the vessel until he washes it three times, for indeed he does not know where his hand has spent the night.
- Sahih Muslim, The book of purity

"Where his hand has spent the night" : an allusion to it sliding onto his rear or his penis, or uncleanliness at all, or any other kind of filth.