Deuteronomy 19:1-3

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.