Deuteronomy 19:1-3

Laws Concerning Manslaughter

19:1 When the Lord your God destroys the nations whose land he is about to give you and you dispossess them and settle in their cities and houses, 19:2 you must set apart for yourselves three cities in the middle of your land that the Lord your God is giving you as a possession. 19:3 You shall build a roadway and divide into thirds the whole extent of your land that the Lord your God is providing as your inheritance; anyone who kills another person should flee to the closest of these cities.

Deuteronomy 19:9

19:9 and then you are careful to observe all these commandments I am giving you today (namely, to love the Lord your God and to always walk in his ways), then you must add three more cities to these three.