Numbers 35:31-33

Amplified Bible

³¹ Moreover, you shall not accept a ransom [in exchange] for the life of a murderer guilty and sentenced to death; but he shall certainly be put to death. ³² You shall not accept a ransom for him who has escaped to his city of refuge, so that he may return to live in his [own] land before the death of the high priest. ³³ So you shall not pollute and defile the land in which you live; for [the shedding of innocent] blood pollutes and defiles the land. No atonement (expiation) can be made for the land for the [innocent] blood shed in it, except by the blood (execution) of him who shed it.