Deuteronomy 19:4-5

This shall be the law of the slayer that fleeth, whose life is to be saved: He that killeth his neighbor ignorantly, and who is proved to have had no hatred against him yesterday and the day before:

But to have gone with him to the wood to hew wood, and in cutting down the tree the axe slipped out of his hand, and the iron slipping from the handle struck his friend, and killed him: he shall flee to one of the cities aforesaid, and live: