Deuteronomy 19:2-9

2you are to set apart three cities for yourself within the land He is giving you to possess.

3You are to prepare the way for yourself and divide into three parts the borders of your land that Adonai your God enables you to inherit, so that anyone who kills may flee there.

4Now this is the case of the one who kills, who may flee there and live—whoever kills his neighbor unintentionally and did not hate him in the past—

5as when someone goes into the forest with his neighbor to chop wood, and his hand takes a swing with the axe to cut down the tree, and the iron flies off the wood and hits his neighbor so that he dies. He may flee to one of these cities and live.

6Otherwise if the way is long, the avenger of the blood, while his heart is hot, may chase the manslayer, catch up to him and strike him dead. Yet no death sentence is on him, since he did not hate him in the past.

7Therefore I am commanding you—set apart three cities for yourself.

8“Suppose Adonai your God enlarges your territory, as He has sworn to your fathers, and He gives you all the land that He promised to give to your fathers—

9when you take care to do all this mitzvah that I am commanding you today, to love Adonai your God and to always walk in His ways. Then you are to add three more cities for yourself, besides these three.