Musnad Ahmad 30:320

Abu Mu'awiyah narrated to us, Al-A'mash narrated to us, from Abu Salih, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If one of you wakes up from the night, let him not put his hand into the vessel until he washes it three times, for he does not know where his hand spent the night."