Abu Dawud Book 1, Hadith Number 74.
Chapter : Performing ablution with water left over after a dog has drunk of it.
Narated By Ibn Mughaffal : The Messenger of Allah (PBUH) ordered the killing of the dogs, and then said: Why are they (people) after them (dogs)? – and then granted permission (to keep) the dog for hunting and for (the security) of the herd, and said: When the dog licks the utensil wash it seven times, and rub it with earth the eight time.
Abu Dawud said: Ibn Mughaffal narrated in a similar way.