Deuteronomy 19:4-10

This is the rule concerning anyone who kills a person and flees there for safety—anyone who kills a neighbor unintentionally, without malice aforethought. For instance, a man may go into the forest with his neighbor to cut wood, and as he swings his ax to fell a tree, the head may fly off and hit his neighbor and kill him. That man may flee to one of these cities and save his life. Otherwise, the avenger of blood might pursue him in a rage, overtake him if the distance is too great, and kill him even though he is not deserving of death, since he did it to his neighbor without malice aforethought. This is why I command you to set aside for yourselves three cities. If the LORD your God enlarges your territory, as he promised on oath to your ancestors, and gives you the whole land he promised them, because you carefully follow all these laws I command you today—to love the LORD your God and to walk always in obedience to him—then you are to set aside three more cities. Do this so that innocent blood will not be shed in your land, which the LORD your God is giving you as your inheritance, and so that you will not be guilty of bloodshed.