Deuteronomy 19:4; Deuteronomy 19:5

4 "This is the provision for the manslayer, who by fleeing there may save his life. If anyone kills his neighbor unintentionally without having hated him in the past--

5 as when someone goes into the forest with his neighbor to cut wood, and his hand swings the axe to cut down a tree, and the head slips from the handle and strikes his neighbor so that he dies--he may flee to one of these cities and live,