Musnad Ahmad 30:697

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-Zuhri, from Ibn al-Musayyib, that Abu Hurayrah said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you wakes up, he should not put his hand into his vessel—or he said: into his ablution water—until he has washed it three times, for he does not know where his hand spent the night."