Sahih Muslim
The Book of Purification (Kitab Al-Taharah)
Book Number: 002 / Hadith Number: 550
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.