A husband and his wife were driving in a convertible. Suddenly, the husband kills the wife with a knife and clears the evidence and throws the knive into a nearby ravine. There were no witnesses of this incident. The next morning, the police find the body and call the husband. They told him his wife is dead and told him to come to the crime scene right away. When he comes, he is arrested on the spot.
How did the police find out?