Deuteronomy 19:11 (KJV)

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:

That innocent blood be not shed in thy land, which the Lord thy God giveth thee for an inheritance, and so blood be upon thee.

But if any man hate his neighbour, and lie in wait for him, and rise up against him, and smite him mortally that he die, and fleeth into one of these cities:

Then the elders of his city shall send and fetch him thence, and deliver him into the hand of the avenger of blood, that he may die.

Thine eye shall not pity him, but thou shalt put away the guilt of innocent blood from Israel, that it may go well with thee.