A man murders his wife with a knife in their car. Nobody is around to see this. 

He throws her out of the car being careful not to leave any fingerprints on her body. 

Next he throws the knife off of a cliff into a gorge where it will never be found and he goes home. 

An hour later the police call him and tell him that his wife has been murdered and he needs to come to the scene of the crime immediately. 

As soon as he arrives, he is arrested.

Question: How did they know he did it?