Deuteronomy 19:5-9

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.

Therefore I command you, saying, ‘You shall set aside for yourself three cities.’

“And if the Lord your God enlarges your territory, just as He has sworn to your fathers, and gives you all the land that He promised to give your fathers—

if you carefully follow all of this commandment which I am commanding you today, to love the Lord your God, and to walk in His ways always—then you shall add three more cities for yourself, besides these three.