Numbers 35:31-33

New Revised Standard Version, Anglicised Catholic Edition

³¹ Moreover, you shall accept no ransom for the life of a murderer who is subject to the death penalty; a murderer must be put to death. ³² Nor shall you accept ransom for one who has fled to a city of refuge, enabling the fugitive to return to live in the land before the death of the high priest. ³³ You shall not pollute the land in which you live; for blood pollutes the land, and no expiation can be made for the land, for the blood that is shed in it, except by the blood of the one who shed it.