Deuteronomy 19:2-9

21you shall set aside three cities for yourself in the midst of your land, which the Lord your God gives you to 2possess. 3You shall prepare the 3roads for yourself, and divide into three parts the territory of your land which the Lord your God will give you as a possession, 4so that any manslayer may flee there.

4“5Now this is the case of the manslayer who may flee there and live: when he 6kills his friend 7unintentionally, 8not hating him previously-- 5as when a man goes into the forest with his friend to cut wood, and his hand 9swings the axe to cut down the tree, and the iron head slips off the 10handle and 11strikes his friend so that he dies--he may flee to one of these cities and live; 6otherwise the avenger of blood might pursue the manslayer 12in the heat of his anger, and overtake him, because the way is long, and 13take his life, though he was not deserving of death, since he had not hated him previously. 7Therefore, I command you, saying, ‘You shall set aside three cities for yourself.’

8“If the Lord your God 14enlarges your territory, just as He has sworn to your fathers, and gives you all the land which He 15promised to give your fathers-- 9if you 16carefully observe all this commandment which I command you today, 17to love the Lord your God, and to walk in His ways always--18then you shall add three more cities for yourself, besides these three.