Sahih Muslim Book 2, Hadith Number 544.
Chapter : It is undesirable to put one’s hand in the utensil before washing it.
Abu Huraira reported: The Apostle of Allah (may peace be upon him) said: When anyone amongst you wakes up from sleep, he should wash his hands three times before putting it in the utensil, for he does not know where his hand was during the night.