Deuteronomy 19:1-3, 8-10

When GOD, your God, throws the nations out of the country that GOD, your God, is giving you and you settle down in their cities and houses, you are to set aside three easily accessible cities in the land that GOD, your God, is giving you as your very own. Divide your land into thirds, this land that GOD, your God, is giving you to possess, and build roads to the towns so that anyone who accidentally kills another can flee there.

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.