Numbers 35:30-33

30Everyone killing anyone shall be put to death as a murderer only on the testimony of more than one witness. No one is to be put to death on the testimony of only one witness.

31“You are not to accept ransom for the life of a murderer. He deserves to die and he is to die.

32Do not receive a ransom for one who has fled to his city of refuge, allowing him to return to his land before the kohen gadol ’s death.

33You are not to pollute the land in which you are. Blood pollutes the land, and no atonement can be made for land polluted by bloodshed except by the blood of the one who sheds it.