Sahih Bukhari Volume 3, Book 39, Hadith Number 515.

Narated By 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 hunting.” Narrated Abu Hazim from Abu Huraira: The Prophet said, “A dog for guarding cattle or for hunting.”