Abu Dawud
Purification (Kitab Al-Taharah)
Book Number: 001 / Hadith Number: 74
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.