Hadith: If anyone keeps a dog that is neither for hunting nor for guarding livestock, their reward (for good deeds) will be reduced by two Qirāts per day.

عن عبد الله بن عمر رضي الله عنهما قال: سمعت رسول الله صلى الله عليه وسلم يقول: «مَن اقْتَنَى كَلْبًا -إلا كلبَ صَيْدٍ، أو مَاشِيَةٍ- فإنه يَنْقُصُ من أَجْرِهِ كل يوم قِيرَاطَانِ». قال سالم: وكان أبو هريرة يقول: «أو كلبَ حَرْثٍ»، وكان صَاحِبَ حَرْثٍ.
[صحيح] - [متفق عليه]

‘Abdullāh ibn ‘Umar (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "If anyone keeps a dog that is neither for hunting nor for guarding livestock, their reward (for good deeds) will be reduced by two Qirāts per day." Sālim said that Abu Hurayrah, who had a farm, used to say: "Or a farm dog."
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

A dog is a despicable filthy animal, and that is why Islamic Shariah forbade owning it because of the harms and bad consequences resulting from keeping it. Examples of these harms include the following: The honorable and obedient angels turn away from the house where a dog is kept; dogs frighten and terrify people; dogs are a source of impurity and harm; and owning a dog is a sign of foolishness. Therefore, anyone who keeps a dog their rewards of good deeds will decrease considerably every day. This amount is translated into two Qirāts, which only Allah knows how big they are, because such a person insists on disobeying Allah by keeping a dog. However, if there is a need for keeping a dog, it is allowed to do so in three cases: 1- Guarding livestock against wolves and thieves. 2- Guarding crops. 3- Hunting animals. A Muslim is allowed to keep a dog for any of these purposes, and is no longer blamed for doing so.