I had no idea that keeping a dog was a sin for muslims? What's the reasoning?
Well technically it's not a sin nor is it forbidden. But, let's say it is 'disapproved' of by the majority of Muslims unless it is for a good reason such as herding, farming, hunting, guide dogs etc...
Having said that though a great many Muslims take no notice of that. We had a dog when I was a boy and in fact it was my Muslim father who bought it for us. Plus I know other practising Muslims who have dogs.
Below is a good summary of the reasons why keeping dogs is generally disapproved by most Muslims.
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Most Muslim scholars agree that the saliva of a dog is ritually impure, and that contact with a dog's saliva requires one to wash seven times. This ruling comes from the hadith:
The Prophet, peace be upon him, said: "If a dog licks the vessel of any one of you, let him throw away whatever was in it and wash it seven times." (Reported by Muslim)
It is to be noted, however, that one of the major Islamic schools of thought (Maliki) indicates that its not a matter of ritual cleanliness, but simply a common-sense method way to prevent the spread of disease.
There are several other hadith, however, which warn of consequences for dog-owners:
The Prophet, peace be upon him, said: "Whoever keeps a dog, his good deeds will decrease every day by one qeeraat (a unit of measurement), unless it is a dog for farming or herding." In another report, it is said: "... unless it is a dog for herding sheep, farming or hunting." (Reported by al-Bukhaari)
The Prophet, peace be upon him, said: "Angels do not enter a house wherein there is a dog or an animate picture."
Many Muslims base the prohibition against keeping a dog in one's home, except for the case of working or service dogs, on these traditions.http://islam.about.com/od/islamsays/a/Dogs-In-Islam.htm