Deuteronomy 19:1-3

“When the LORD your God has cut off the nations whose land the LORD your God is giving you, and you dispossess them and dwell in their cities and in their houses, you shall separate three cities for yourself in the midst of your land which the LORD your God is giving you to possess. You shall prepare roads for yourself, and divide into three parts the territory of your land which the LORD your God is giving you to inherit, that any manslayer may flee there.