Volume 4 · Book 54 · Hadith 541
Narrated Abu Huraira: Allah's Apostle said, "If somebody keeps a dog, he loses one Qirat (of the reward) of his good deeds everyday, except if he keeps it for the purpose of agriculture or for the protection of livestock. "
— Sahih Bukhari, Volume 4, Book 54: Beginning of Creation, Hadith #541