Deuteronomy 19:2-6

you shall set aside for yourself three cities in the midst of your land which the Lord your God is giving you to possess.

You shall prepare the roads for yourself, and divide into three regions the territory of your land which the Lord your God will give you as an inheritance, so that anyone who commits manslaughter may flee there.

“Now this is the case of the one who commits manslaughter, who may flee there and live: when he kills his friend unintentionally, not hating him previously—

as when a person goes into the forest with his friend to cut wood, and his hand swings the axe to cut down the tree, and the iron head slips off the handle and strikes his friend so that he dies—he may flee to one of these cities and live.

Otherwise, the avenger of blood might pursue him in the heat of his anger, and overtake him because the way is long, and take his life, though he was not sentenced to death since he had not hated him previously.