Staley Fleming's Hallucination

One man said to a physician that he was disturbed by hallucinations. Every night he awoke and saw in his room a big black dog. It seemed to him sinister. The doctor said that the patient looked allright, and suggested that it could be just a dream. But the man reassured the physician that the dog was indeed. After that the doctor said that the dog which he described was fit the dog which belonged to the man’s enemy. He was dead about three years ago. He had been stabbed to death and his body was found in a forest. There wasn’t any piece of evidence at the place of crime… Read the story till the end to know what the connection was with the murder and hallucinations!