Musnad Ahmad 834:23

Muhammad ibn Ja'far narrated to us, 'Awf narrated to us, from al-Hasan, from 'Abdullah ibn Mughaffal, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Were it not that dogs are a nation among the nations, I would have commanded that they be killed. But kill every pure black one among them."