Numbers 35:31-33

Moreover ye shall take no ransom for the life of a murderer, that is guilty of death; but he shall surely be put to death. And ye shall take no ransom for him that is fled to his city of refuge, that he may 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 polluteth the land; and no expiation can be made for the land for the blood that is shed therein, but by the blood of him that shed it.