Sahih Bukhari Volume 3, Book 39, Hadith Number 516.

Narated By As-Sa’ib bin Yazid : Abu Sufyan bin Abu Zuhair, a man from Azd Shanu’a and one of the companions of the Prophet said, “I heard Allah’s Apostle saying, ‘If one keeps a dog which is meant for guarding neither a farm nor cattle, one Qirat of the reward of his good deeds is deducted daily.” I said, “Did you hear this from Allah’s Apostle?” He said, “Yes, by the Lord of this Mosque.”