Hadhrat Abu Talha (Radhiyallaahu Ánhu) reports that Rasulullah (Sallallaaahu Álayhi Wasallam) said, "Angels do not enter a house wherein there is a dog or an animate picture. (Sahih Bukhari Hadith no. 2986)
Hadhrat Ibn Abbas (Radhiyallaahu Ánhuma) reports from Hadhrat Maimoona (Radhiyallaahu Ánha) that once Rasulullah (Sallallaaahu Álayhi Wasallam) became sad; and said that Jibra'eel (Álayhis Salaam) promised to meet him at night but did not turn up. "By Allah what has kept him back," said the Prophet (Sallallaaahu Álayhi Wasallam). Then he realised a puppy was under his bed. He ordered that the puppy be removed and the area be sprinkled with water. In the afternoon when Jibra'eel (Álayhi Salaam) came, Rasulullah (Sallallaaahu Álayhi Wasallam) enquired as to the delay. Jibra'eel (Álayhi Salaam) said that we, the group of Angels do not enter a house wherein there is a dog or pictures. (Sahih Muslim Hadith no.3928)
In the light of these Ahaadith and other narrations it is not permissible to keep dogs as pets. The household is deprived of the Mercy of Allah Taãla.
However, Jurists have stated that it is permissible to keep a dog for security purposes, farming and hunting.
The saliva of a dog is Najis (impure). if it touches the clothes or body, that portion also becomes impure and must be washed.