Deuteronomy 19:3-5

Amplified Bible

³ You shall prepare and maintain for yourself the roads [to these cities], and divide the territory of your land into three parts, so that anyone who kills another unintentionally may escape there [for asylum].

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— ⁵ as [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;