Deuteronomy 19:1-3

When the LORD your God has destroyed the nations whose land he is giving you, and when you have driven them out and settled in their towns and houses, then set aside for yourselves three cities in the land the LORD your God is giving you to possess. Determine the distances involved and divide into three parts the land the LORD your God is giving you as an inheritance, so that a person who kills someone may flee for refuge to one of these cities.