Sahih Muslim
The Book of Purification (Kitab Al-Taharah)
Book Number: 002 / Hadith Number: 546
Abu Huraira reported the Messenger of Allah (may peace be upon him) to have said: When a dog licks a utensil belonging to any one of you, (the thing contained in it) should be thrown away and then (the utensil) should be washed seven times.