Numbers 35:13

“Speak to the children of Israel, and tell them, ‘When you pass over the Jordan into the land of Canaan,

then you shall appoint for yourselves cities to be cities of refuge for you, that the man slayer who kills any person unwittingly may flee there.

The cities shall be for your refuge from the avenger, that the man slayer not die until he stands before the congregation for judgment.

You shall give three cities beyond the Jordan, and you shall give three cities in the land of Canaan. They shall be cities of refuge.

These six cities shall be refuge for the children of Israel, for the stranger, and for the foreigner living among them, that everyone who kills any person unwittingly may flee there.

“‘But if he struck him with an instrument of iron, so that he died, he is a murderer. The murderer shall surely be put to death.