Deuteronomy 19:9

5as when a man goes into the forest with his neighbor to chop wood and his hand swings the ax to cut down the tree, and the head slips from the handle and hits his neighbor so that he dies—he shall flee to one of these cities and live.

6Otherwise, the avenger of blood might pursue the man slayer while hot anger is in his heart and overtake him, because the way is long, and strike him mortally, even though he was not worthy of death, because he didn’t hate him in time past.

7Therefore I command you to set apart three cities for yourselves.

8If the LORD your God enlarges your border, as he has sworn to your fathers, and gives you all the land which he promised to give to your fathers;

9and if you keep all this commandment to do it, which I command you today, to love the LORD your God, and to walk ever in his ways, then you shall add three cities more for yourselves, in addition to these three.

10This is so that innocent blood will not be shed in the middle of your land which the LORD your God gives you for an inheritance, leaving blood guilt on you.

11But if any man hates his neighbor, lies in wait for him, rises up against him, strikes him mortally so that he dies, and he flees into one of these cities;

12then the elders of his city shall send and bring him there, and deliver him into the hand of the avenger of blood, that he may die.

13Your eye shall not pity him, but you shall purge the innocent blood from Israel that it may go well with you.