Sahih Muslim Book 004, Hadith Number 1032.
Chapter : The excellence of the prayer and the way it is begun and the excellence of Ruku' and moderation in it and prostration and moderation in it, etc.
Abu Dharr reported: The Messenger of 'Allah (may peace be upon him) said: When any one of you stands for prayer and there is a thing before him equal to the back of the saddle that covers him and in case there is not before him (a thing) equal to the back of the saddle, his prayer would be cut off by (passing of an) ass, woman, and black Dog. I said: O Abu Dharr, what feature is there in a black dog which distinguish it from the red dog and the yellow dog? He said: O, son of my brother, I asked the Messenger of Allah(may peace be upon him) as you are asking me, and he said: The black dog is a devil.