Sahih Muslim Book 2, Hadith Number 550.
Chapter : Instructions pertaining to the licking of a dog.
Hammam b. Munabbih reported: Of the a hadith narrated by Abu Huraira from Muhammad, the Messenger of Allah (may peace be upon him), one is this: The Messenger of Allah (may peace be upon him) said: The purification of the utensil belonging to one amongst you, after it is licked by a dog, lies in washing it seven times.