Abu Dawud Book 1, Hadith Number 71.

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

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.