DOGS (Arabic kalb, pl. kilab; Heb. ) are unclean animals according to a tradition by Abu Hurairah, Muhammad said that when a dog drinks in a vessel, it must be washed seven times, and that the first cleansing should be with earth. (Mishkat, book iii c I pt. 1)
“Most people believe that when a dog howls near a house it forebodes death, for it is said a dog can distinguish the awful form of Azra’il, the Angel of Death.” (Burton’s Arabia, vol I p 290)
Ibn ‘Umr says that dogs used to come into the Masjid at Makkah in the time of the Prophet, but the Companions never purified the mosque when the dog was dry.
The Imam Abu Yusuf holds that the sale of a dog that bites is unlawful, whilst the Imam ash-Shafi’i has siad that the sale of a dog is absolutely illegal, because the Prophet said that the wages of whoredom and the price of a dog are forbidden. Abu Hanifah holds that dogs which are trained to hunt or watch may be lawfully sold. (Hamilton’s Hidayah, vol ii p 543.)
It is lawful to hunt with a trained dog and the sign of a dog being trained is that he catches game three times without killing it. The dog must be let slip with the ejaculation Bismillahi ‘llahi Akbar! “In the name of God, the great God!” When all game seized by him becomes lawful food. This custom is founded upon a verse in the Qur’an, Surah v. 6: “Lawful for you are all good things and what ye have taught the beasts of prey to catch, training them like dogs; ye teach them as God taught you. And mention the name of God over it.”
Rules for hunting with dogs will be found in Hamilton’s Hidayah, vol iv p 170.

Based on Hughes, Dictionary of Islam