Sahih Muslim Book 2, Hadith Number 551.

Ibn Mughaffal reported: The Messenger of Allah (may peace be upon him) ordered killing of the dogs, and then said: What about them, i.e. about other dogs? and then granted concession (to keep) the dog for hunting and the dog for (the security) of the herd, and said: When the dog licks the utensil, wash it seven times, and rub it with earth the eighth time.