Numbers 35:30-34

³⁰Whoever kills any person, the murderer shall be slain at the mouth of witnesses: but one witness shall not testify against any person that he die. ³¹Moreover you shall take no ransom for the life of a murderer who is guilty of death; but he shall surely be put to death. ³²You shall take no ransom for him who is fled to his city of refuge, that he may come again to dwell in the land, until the death of the priest. ³³So you shall not pollute the land in which you are: for blood, it pollutes the land; and no expiation can be made for the land for the blood that is shed therein, but by the blood of him who shed it. ³⁴You shall not defile the land which you inhabit, in the midst of which I dwell: for I, Yahweh, dwell in the midst of the children of Israel.