Deuteronomy 19:3-5

New Revised Standard Version, Anglicised

³ You shall calculate the distances^[a] and divide into three regions the land that the Lord your God gives you as a possession, so that any homicide can flee to one of them.

⁴ Now this is the case of a homicide who might flee there and live, that is, someone who has killed another person unintentionally when the two had not been at enmity before: ⁵ Suppose someone goes into the forest with another to cut wood, and when one of them swings the axe to cut down a tree, the head slips from the handle and strikes the other person who then dies; the killer may flee to one of these cities and live.