Deuteronomy 19:4-5

4“1Now this is the case of the one who commits manslaughter, who may flee there and live: when he 2kills his friend 3unintentionally, 4not hating him previously--

5as when a person goes into the forest with his friend to cut wood, and his hand 5swings the axe to cut down the tree, and the iron head slips off the 6handle and 7strikes his friend so that he dies--he may flee to one of these cities and live.