Deuteronomy 19:5-7

Complete Jewish Bible

⁵ An example would be if a man goes into the forest with his neighbor to cut wood and takes a stroke with the axe to fell a tree, but the head of the axe flies off the handle, hits his neighbor and kills him. Then he is to flee to one of these cities and live there. ⁶ Otherwise the next-of-kin avenger, in the heat of his anger, may pursue the killer, overtake him because the distance [to the city of refuge] is long, and strike him dead — even though he didn’t deserve to die, inasmuch as he hadn’t hated him in the past. ⁷ This is why I am ordering you to set aside for yourselves three cities.