Musnad Ahmad 834:29

Muhammad ibn Ja'far narrated to us, 'Awf narrated to us, from al-Hasan, from 'Abdullah ibn Mughaffal, that the Messenger of Allah (may Allah bless him and grant him peace) said: "Whoever keeps a dog, except for a hunting dog, or a sheep dog, or a farm dog, then indeed, a qirat of his deeds decreases every day."