Deuteronomy 19:1-3

“When the Lord your God cuts off the nations whose land the Lord your God is giving you, and you dispossess them and settle in their cities and in their houses,

you shall set aside for yourself three cities in the midst of your land which the Lord your God is giving you to possess.

You shall prepare the roads for yourself, and divide into three regions the territory of your land which the Lord your God will give you as an inheritance, so that anyone who commits manslaughter may flee there.