Numbers 35:9-13

Then the LORD said to Moses: “Speak to the Israelites and say to them: ‘When you cross the Jordan into Canaan, select some towns to be your cities of refuge, to which a person who has killed someone accidentally may flee. They will be places of refuge from the avenger, so that anyone accused of murder may not die before they stand trial before the assembly. These six towns you give will be your cities of refuge.