Sahih Bukhari
Beginning of Creation
Volume: 004 / Book Number: 054 / Hadith Number: 542
Narrated Sufyan bin Abi Zuhair Ash-Shani:
That he heard Allah's Apostle saying, "If somebody keeps a dog that is neither used for farm work nor for guarding the livestock, he will lose one Qirat (of the reward) of his good deeds everyday."