Deuteronomy 19:2-9

2then you are to set apart for yourselves three cities within the land that the LORD your God is giving you to possess. 3You are to build roads for yourselves1 and divide into three regions the land that the LORD your God is giving you as an inheritance, so that any manslayer can flee to these cities.

4Now this is the situation regarding the manslayer who flees to one of these cities to save his life, having killed his neighbor accidentally, without intending to harm him: 5If he goes into the forest with his neighbor to cut timber and swings his axe to chop down a tree, but the blade flies off the handle and strikes and kills his neighbor, he may flee to one of these cities to save his life.

6Otherwise, the avenger of blood might pursue the manslayer in a rage, overtake him if the distance is great, and strike him dead though he did not deserve to die, since he did not intend any harm. 7This is why I am commanding you to set apart for yourselves three cities.

8And if the LORD your God enlarges your territory, as He swore to your fathers, and gives you all the land He promised them, 9and if you carefully keep all these commandments I am giving you today, loving the LORD your God and walking in His ways at all times, then you are to add three more cities to these three.