Musannaf Ibn Abi Shayba 35:190

Abu Mu'awiyah narrated to us, from al-A'mash, from Abu Razin, from Abu Hurayrah, who said: The Prophet (peace and blessings of Allah be upon him) said: "When one of you wakes up at night, let him not dip his hand into the vessel until he has washed it three times, for he does not know where his hand spent the night."