Jami` at-Tirmidhi 338

Abu Dharr said that :Allah's Messenger said: "When a man performs Salat, and there is nothing in front of him like the post of a saddle, or a camel saddle, then his Salat is severed by (passing of) a black dog, a woman, and a donkey." It was said to Abu Dharr: "What is the problem with the black dog rather than the red or white one?" He said: "O my nephew! I asked Allah's Messenger just as you have asked me. He said: 'The black dog is a Shaitan (devil)