Musnad Ahmad 27:944

Hassan narrated to us, Ibn Lahi'ah narrated to us, Abu al-Zubayr narrated to us: I asked Jabir about keeping a dog. He said: Ibn Umar informed me that he heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "Whoever keeps one, two qirats are deducted from his reward every day."