Allah's Apostle said, Whoever keeps a dog, one Qirat of the reward of...

Hadith 3:515

Narrated Abu Huraira:

Allah's Apostle said, "Whoever keeps a dog, one Qirat of the reward of his good deeds is deducted daily, unless the dog is used for guarding a farm or cattle." Abu Huraira (in another narration) said from the Prophet, "unless it is used for guarding sheep or farms, or for dog for guarding cattle or for hunting."