Numbers 35:31-33

Moreover, you shall accept no ransom for the life of a murderer, who is guilty of death, but he shall be put to death. And you shall accept no ransom for him who has fled to his city of refuge, that he may return to dwell in the land before the death of the high priest. You shall not pollute the land in which you live, for blood pollutes the land, and no atonement can be made for the land for the blood that is shed in it, except by the blood of the one who shed it.