Deuteronomy 19:6-8

American Standard Version

⁶ 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. ⁷ Wherefore I command thee, saying, Thou shalt set apart three cities for thee. ⁸ And if Jehovah thy God enlarge thy border, as he hath sworn unto thy fathers, and give thee all the land which he promised to give unto thy fathers;