Deuteronomy 19:4-7

4“Here is the law concerning a case of someone who kills a person and flees there to save his life, having killed his neighbor accidentally without previously hating him:

5If, for example, he goes into the forest with his neighbor to cut timber, and his hand swings the ax to chop down a tree, but the blade flies off the handle and strikes his neighbor so that he dies, that person may flee to one of these cities and live.

6Otherwise, the avenger of blood in the heat of his anger might pursue the one who committed manslaughter, overtake him because the distance is great, and strike him dead. Yet he did not deserve to die, since he did not previously hate his neighbor.

7This is why I am commanding you to set apart three cities for yourselves.