Deuteronomy 19:6

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 neighbour ignorantly, whom he hated not in time past;

5 As when a man goeth into the wood with his neighbour 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 neighbour, 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.

8 And if the LORD thy God enlarge thy coast, as he hath sworn unto thy fathers, and give thee all the land which he promised to give unto thy fathers;

9 If thou shalt keep all these commandments to do them, which I command thee this day, to love the LORD thy God, and to walk ever in his ways; then shalt thou add three cities more for thee, beside these three: