Abu Dawud
Purification (Kitab Al-Taharah)
Book Number: 001 / Hadith Number: 105
Narated By Abu Hurairah : I heard the Messenger of Allah (pbuh) say: "When any one of you wakes from sleep, he should not put his hand in the utensil until he washes it three times, for none of you knows where his hand remained during the night or where it went round.