Deuteronomy 19:4-7

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.