Deuteronomy 19:2-4
2
you shall set apart three cities for you in the midst of your land, which Yahweh your God gives you to possess it.
3
You shall prepare you the way, and divide the borders of your land, which Yahweh your God causes you to inherit, into three parts, that every manslayer may flee there.
4
This is the case of the manslayer, that shall flee there and live: whoever kills his neighbor unawares, and didn't hate him in time past;