Deuteronomy 19:4-7

4“Now this is the case of the offender (manslayer) who may escape there and live [protected from vengeance]: when he kills his neighbor unintentionally, not having hated him previously—

5as [for example] when a man goes into the forest with his neighbor to cut wood, and his hand swings the axe to cut down the tree, but the iron head slips off the wooden handle and hits his companion and he dies—the offender may escape to one of these cities and live;

6otherwise the avenger of blood might pursue the offender in the heat of anger, and overtake him, because it is a long way, and take his life, even though he did not deserve to die, since he did not hate his neighbor beforehand.

7Therefore, I command you, saying, ‘You shall set aside three cities [of refuge] for yourself.’