Deuteronomy 19:6-12

19:6 Otherwise the blood avenger will chase after the killer in the heat of his anger, eventually overtake him, and kill him, though this is not a capital case since he did not hate him at the time of the accident. 19:7 Therefore, I am commanding you to set apart for yourselves three cities. 19:8 If the Lord your God enlarges your borders as he promised your ancestors and gives you all the land he pledged to them, 19:9 and then you are careful to observe all these commandments I am giving you today (namely, to love the Lord your God and to always walk in his ways), then you must add three more cities to these three. 19:10 You must not shed innocent blood in your land that the Lord your God is giving you as an inheritance, for that would make you guilty. 19:11 However, suppose a person hates someone else and stalks him, attacks him, kills him, and then flees to one of these cities. 19:12 The elders of his own city must send for him and remove him from there to deliver him over to the blood avenger to die.