Musannaf Ibn Abi Shayba 35:196

Shababa ibn Sawwar narrated to us, from Shu'bah, from Abu al-Tayyah, who said: I heard Mutarrif narrating from Ibn al-Mughaffal that the Messenger of Allah (may Allah bless him and grant him peace) ordered the killing of dogs. And he said: "If a dog licks a vessel, wash it seven times, and rub it the eighth time with earth." And he mentioned that Abu Hanifah said: "It suffices to wash it once."