Numbers 35:31-33

Moreover ye shall take no satisfaction for the life of a murderer, which is guilty of death: but he shall be surely put to death. And ye shall take no satisfaction for him that is fled to the city of his refuge, that he should come again to dwell in the land, until the death of the priest. So ye shall not pollute the land wherein ye are: for blood it defileth the land: and the land cannot be cleansed of the blood that is shed therein, but by the blood of him that shed it.