Deuteronomy 19:4-7

4“1Now this is the case of the manslayer who may flee there and live: when he 2kills his friend 3unintentionally, 4not hating him previously-- 5as when a man 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; 6otherwise the avenger of blood might pursue the manslayer 8in the heat of his anger, and overtake him, because the way is long, and 9take his life, though he was not deserving of death, since he had not hated him previously. 7Therefore, I command you, saying, ‘You shall set aside three cities for yourself.’