Deuteronomy 19:4-5

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;