Deuteronomy 19:1-8

1 The LORD your God will destroy all the nations that are living in the land that he's giving you. You will force them out and live in their cities and houses.
2 When all this is done, set aside three cities in the land that the LORD your God is giving you.
3 Provide a route to each of these cities and divide the land that the LORD your God is giving you into three regions. Whoever kills someone may run to one of these cities.
4 A person who unintentionally kills someone he never hated in the past may run to one of these cities to save his life.
5 Suppose two people go into the woods to cut wood. As one of them swings the ax to cut down a tree, the head flies off the handle, hits, and kills the other person. The one who accidentally killed the other person may run to one of these cities and save his life.
6 Otherwise, in a rage the relative who has the authority to avenge the death will pursue him. If the place is too far away, the relative may catch up with him and take his life even though he didn't deserve the death penalty, because in the past he never hated the person he killed.
7 This is why I'm commanding you to set aside three cities for yourselves.
8 The LORD your God may expand your country's borders as he promised your ancestors with an oath. He may give you the whole land he promised to give them.