All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
The scholars differed in opinion with regard to the dog's urine and its excrements whether what has been affected by them should be cleaned 7 times one of which with the soil, as in the case of its saliva, or just once. Some of them said that whatever is stained by its urine or excrement should be cleaned 7 times and some are of the view that once is enough.
However, the most correct opinion, and in order to be on the safe side, if something pure is stained by the urine, excrement, blood or sweat of the dog, it should be cleaned 7 times one of which with soil, and this is the opinion of the Hanbali and Shaafi'i Schools of jurisprudence.
Finally, we draw the attention of the questioner that it is not permissible to keep a dog except if it is used for hunting, guarding a farm or cattle. The Prophet said: "Whoever keeps a dog, one Qirat of the reward of his good deeds is deducted daily unless the dog is used for guarding a farm, a cattle or for hunting." [Al-Bukhari and Muslim]
Moreover, the permissibility of a farm dog is mentioned in the Hadeeth reported by An-Nasaa'i and which was classified by Imaam Al-Albaani as authentic.
As regards a dog that is used as a guard, it is permissible according to the consensus of the scholars .
Allaah Knows best.