Deuteronomy 19:4-7

And this is the case of the manslayer, that shall flee thither and live: whoso killeth his neighbor unawares, and hated him not in time past; as when a man goeth into the forest with his neighbor to hew wood, and his hand fetcheth a stroke with the axe to cut down the tree, and the head slippeth from the helve, and lighteth upon his neighbor, so that he dieth; he shall flee unto one of these cities and live: lest the avenger of blood pursue the manslayer, while his heart is hot, and overtake him, because the way is long, and smite him mortally; whereas he was not worthy of death, inasmuch as he hated him not in time past. Wherefore I command thee, saying, Thou shalt set apart three cities for thee.