Deuteronomy 19:8-10

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.