It is narrated from Sayyiduna Ibn Mughaffal (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) ordered the killing of dogs, then said: "What concern have I with them?" Then he (peace and blessings be upon him) granted a concession for the hunting dog and the dog of the goats, and said: "When a dog puts its mouth into a vessel, wash it seven times, and scour it with dust the eighth time."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1185