Deuteronomy 19:4-7

4"This is the provision for the manslayer, who by fleeing there may save his life. If anyone kills his neighbor unintentionally without having hated him in the past—

5as when someone goes into the forest with his neighbor to cut wood, and his hand swings the axe to cut down a tree, and the head slips from the handle and strikes his neighbor so that he dies—he may flee to one of these cities and live,

6lest the avenger of blood in hot anger pursue the manslayer and overtake him, because the way is long, and strike him fatally, though the man did not deserve to die, since he had not hated his neighbor in the past.

7Therefore I command you, You shall set apart three cities.