Abu Dawud
Purification (Kitab Al-Taharah)
Book Number: 001 / Hadith Number: 103
Narated By Abu Hurairah : The Messenger of Allah (pbuh) said: When anyone amongst you wakes up from sleep at night, he should not put his hand in the utensil until he washed his hand three times, for he does not know where his hand was during the night.