Sunan Ibn Majah Book 1, Hadith 128
The Messenger of Allah said: "When anyone of you wakes up from sleep, he should not put his hand into the vessel until he has washed it."
Abu Hurairah used to say: "The Messenger of Allah said: 'When anyone of you wakes up from sleep, he should not put his hand into the vessel until he has poured water on it two or three times, for none of you knows where his hand spent the night,'"
"The Messenger of Allah said: 'When anyone of oyu gets up from sleep and wants to perform ablution, he should not put his hand into the vessel he used for ablution until he has washed it, because he does not know where his hand spent the night or where he put it.'"