Abu Dawud
Purification (Kitab Al-Taharah)
Book Number: 001 / Hadith Number: 73
Narated By Abu Hurairah : The Prophet (pbuh) said: When a dog licks a (thing contained in a) utensil you must wash it seven times, using earth (sand) for the seventh time.
Abu Dawud said: This tradition has been transmitted by another chain of narrators in which there is no mention of earth.