Deuteronomy 19:2-4

New Revised Standard Version Catholic Edition

² you shall set apart three cities in the land that the Lord your God is giving you to possess. 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: