Abu Dawud Book 1, Hadith Number 73.

Chapter : Performing ablution with water left over after a dog has drunk of it.

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.