It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Keeping a dog causes a decrease (in reward) of two qirats every day, except for a hunting dog or a dog that guards sheep."
Imam al-Tabarani (may Allah have mercy on him) said: "The decrease means a decrease in the reward of its owner."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13101