Abu Dawud
Purification (Kitab Al-Taharah)
Book Number: 001 / Hadith Number: 71
Narated By Abu Hurairah : The Prophet (pbuh) said: The purification of the utensil belonging to any one of you, after it has been licked by a dog, consists of washing it seven times, using sand in the first instance.
Abu Dawud said: A similar tradition has been narated by Abu Ayyub and Habib b. al-Shahid on the authority of Muhammad.