Deuteronomy 19:5

³  Thou shalt prepare thee a way, and divide the coasts of thy land, which the LORD thy God giveth thee to inherit, into three parts, that every slayer may flee thither.

⁴  And this is the case of the slayer, which shall flee thither, that he may live: Whoso killeth his neighbor ignorantly, whom he hated not in time past;

⁵  As when a man goeth into the wood 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, that he die; he shall flee unto one of those cities, and live:

⁶  Lest the avenger of the blood pursue the slayer, while his heart is hot, and overtake him, because the way is long, and slay him; whereas he was not worthy of death, inasmuch as he hated him not in time past.

⁷  Wherefore I command thee, saying, Thou shalt separate three cities for thee.