Deuteronomy 19:2-4

a you shall set apart three cities for yourselves in the land that the  Lord your God is giving you to possess. You shall measure the distances
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and divide into three parts the area of the land that the  Lord your God gives you as a possession, so that any manslayer can flee to them.

This is the provision for c the manslayer, who by fleeing there may save his life. If anyone kills his neighbor unintentionally without having hated him in the past
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