Numbers 35:30-33

30Whoever shall smite a person mortally, at the mouth of witnesses shall the murderer be put to death; but one witness shall not testify against a person to cause him to die.

31And ye shall take no satisfaction for the life of a murderer, who is guilty of death, but he shall certainly be put to death.

32And ye shall take no satisfaction for him that hath fled to the city of his refuge, that he should come again to dwell in the land, until the death of the priest.

33And ye shall not pollute the land wherein ye are; for blood, it polluteth the land; and there can be no atonement made for the land, for the blood that hath been shed therein, but by the blood of him that shed it.