Musnad Ahmad 30:2961

Waki' narrated to us, he said: Al-A'mash narrated to us, from Abu Salih and Abu Razin, from Abu Hurairah, who attributed it to the Prophet (may Allah bless him and grant him peace)—this is how Al-A'mash said it—he said: "When one of you wakes up from his sleep, 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."