Numbers 35:9-13

Then the Lord spoke to Moses, saying,

“Speak to the sons of Israel and say to them, ‘When you cross the Jordan into the land of Canaan,

then you shall select for yourselves cities to be your cities of refuge, so that the one who commits manslaughter by killing a person unintentionally may flee there.

The cities shall serve you as a refuge from the avenger, so that the one who commits manslaughter does not die until he stands before the congregation for trial.

So the cities which you are to provide shall be six cities of refuge for you.