Musnad Ahmad 430:6

Yahya narrated to us, from Shu'bah, who said: Abu al-Tayyah narrated to us, from Mutarrif, from Ibn Mughaffal, that the Messenger of Allah (peace and blessings of Allah be upon him) ordered the killing of dogs, then he said: "What do they have to do with them?" So he granted a concession for a hunting dog and for a sheep dog. He said: "And if a dog licks a vessel, then wash it seven times, and the eighth time, rub it with earth."