Numbers 35:31-33

Complete Jewish Bible

³¹ Also, you are not to accept a ransom in lieu of the life of a murderer condemned to death; rather, he must be put to death. ³² Likewise, you are not to accept for someone who has fled to his city of refuge a ransom that would allow him to return to his land before the death of the cohen. ³³ In this way you will not defile the land in which you are living. For blood defiles the land, and in this land no atonement can be made for the blood shed in it except the blood of him who shed it.