Musnad Ahmad 33:460

Rawh narrated to us, Ibn Jurayj narrated to us, Abu al-Zubayr informed me that he heard Jabir ibn Abdullah saying: The Prophet, may Allah bless him and grant him peace, commanded us to kill dogs, to the extent that a woman would come from the desert with her dog, and we would kill it. Then the Prophet forbade killing them and said: "You must [kill] the solid black one with two spots, for it is a devil."