Deuteronomy 19:8-10

When GOD, your God, enlarges your land, extending its borders as he solemnly promised your ancestors, by giving you the whole land he promised them because you are diligently living the way I’m commanding you today, namely, to love GOD, your God, and do what he tells you all your life; and when that happens, then add three more to these three cities so that there is no chance of innocent blood being spilled in your land. GOD, your God, is giving you this land as an inheritance—you don’t want to pollute it with innocent blood and bring guilt upon yourselves.