Vol 1, Book 4. Ablutions (Wudu'). Hadith 173.
Narrated By Abu Huraira: Allah's Apostle said, "If a DOG drinks from the utensil of anyone of you it is essential to wash it seven times."
Sheikh Ahmad Kutty, a senior lecturer and Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
"A dog can be owned for purposes such as the following:
1. A trained dog for hunting. Remember in Islam we are only allowed to hunt for food; there is no such thing as hunting for fun, for we are not allowed to kill or torture animals or drive them out of their habitats for the fun of it.
2. A trained dog as a guide. This would be the case if a person is blind and he/she has no choice but to keep a dog for essential services. In this case, it is permissible for him/her to keep a dog inside the house once it has been trained for service, but it is still recommended that the dog have its own sleeping arrangement.
3. A dog trained for police duties.
4. A guard dog to guard houses or property.
5. A dog used by farmers to shepherd cattle and sheep.
We are, however, not allowed to keep a dog as a pet, since it is not a very clean animal. How often have you come across the nasty sight of dogs taken for a walk licking their own excrement? Isn't it disgusting to see their owners kissing the mouth of such animals after such incidents as if nothing happened? Would you kiss the mouth of your own baby if he were to do the same? Think how many parasites and microbes may be hiding in their mouths.
The Prophet (peace and blessings be upon him) has spared us from being contaminated by such filth when he ordered us to stay clear of the saliva of dogs. If we ever come into contact with a dog's saliva we must wash the spot seven times, the first of which should be with sand or dirt. It is also possible to use a bacterial soap instead of sand or dirt.
In conclusion: Don't contemplate taking a dog home as a pet. If, however, you do need to keep a dog for any of the reasons given above, then you may do so. But take every precaution not to have contact with its saliva, and also arrange for a separate living space."
Wassalam and Allah Almighty knows best.
islamicity.com got it from Imam