Sunan Ibn Majah Book 1, Hadith 126
"I used to help the Messenger of Allah perform ablution, when I was standing and he was sitting."
"I poured water for the Prophet on journeys and as a resident, when he performed ablution."
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,'"