Musnad Ahmad 834:25

'Abd al-Samad narrated to us, al-Hakam ibn 'Atiyyah narrated to us, he said: I asked al-Hasan about a man who keeps a dog in his house. He said: 'Abdullah ibn Mughaffal narrated to me that the Messenger of Allah (ﷺ) said: "Whoever keeps a dog, his reward decreases by one qirat every day."