Numbers 35:12-16

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.

You shall provide three cities across the Jordan, and three cities in the land of Canaan; they are to be cities of refuge.

These six cities shall be a refuge for the sons of Israel, for the stranger, and for the foreign resident among them; so that anyone who kills a person unintentionally may flee there.

‘But if he struck him with an iron object, so that he died, he is a murderer; the murderer must be put to death.