Sunan Abi Dawud

Narrated Ibn Mughaffal: The Messenger of Allaah (sal Allaahu alayhi wa sallam) ordered the killing of the dogs, and then said: Why are they (people) after them (dogs)? and then granted permission (to keep) 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 eighth time. Abu Dawud said : Ibn Mughaffal narrated in a similar way