Musnad Ahmad 27:1325

Muhammad ibn Ubayd narrated to us, Ubayd Allah narrated to us, from Nafi’, from Ibn Umar, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever acquires a dog, except for a dog for livestock or a hunting dog, two qirats of his deeds will be deducted every day." And he used to order that dogs be killed.