Numbers 35:30-33

30‘1If anyone kills a person, the murderer shall be put to death on the 2testimony of witnesses, but 3no person shall be put to death on the testimony of only one witness.

31Moreover, you shall not accept a ransom for the life of a murderer who is 4condemned to death, but he must be put to death.

32And you shall not accept a ransom for one who has fled to his city of refuge, so that he may return to live in the land 5before the death of the priest.

336So you shall not defile the land in which you live; for blood defiles the land, and no atonement can be made for the land for the blood that is shed on it, except 7by the blood of the one who shed it.