From Sahih Muslim (Hadith 652):
Hammam b. Munabbih reported:Of the a hadith narrated by Abu Huraira from Muhammad, the Messenger of Allah (ﷺ), one is this: The Messenger of Allah (ﷺ) said: The purification of the utensil belonging to one amongst you, after it is licked by a dog, lies in washing it seven times
Reference: Muslim 652