משנה, מכות ב:א

You shall settle for yourselves cities, cities of refuge they will be for you, and a murderer who hits [מכה] a person unintentionally will flee there. The cities shall serve you as a refuge from the avenger, so that the manslayer may not die unless he has stood trial before the assembly. The towns that you thus assign shall be six cities of refuge in all. Three cities shall be designated beyond the Jordan, and the other three shall be designated in the land of Canaan: they shall serve as cities of refuge. These six cities shall serve the Israelites and the resident aliens among them for refuge, so that anyone who kills a person unintentionally may flee there.

But if he pushed him without malice aforethought or hurled any object at him unintentionally, or inadvertently dropped upon him any deadly object of stone, and death resulted—though he was not an enemy of his and did not seek his harm—in such cases the assembly shall decide between the slayer and the blood-avenger. The assembly shall protect the manslayer from the blood-avenger, and the assembly shall restore him to the city of refuge to which he fled, and there he shall remain until the death of the high priest who was anointed with the sacred oil.

These are the ones who go into exile [to an Ir Miklat]: one who kills a person accidentally.If he was pushing a roller [on a roof] and it fell on him [a bystander] and killed him, or if he was lowering a barrel [from a roof] and it fell and killed him, or if he was descending a ladder and he fell and killed him--he goes into exile.However, if he was pulling a roller and it fell and killed the person, or if he was raising a barrel and the rope tore and it fell and killed a person, or if he was climbing a ladder and he fell and killed a person--he does not go into exiled.This is the principle: any who allowed the object to descend goes into exile; and any who did not intend to allow the object to descend, does not go into exile.If the blade came off the handle and killed--Rebbi says, he does not go into exile; the [other] Sages say, he goes into exile. If it[?] came from the wood being chopped --Rebbi says, he goes into exile; the [other] Sages say, he does not go into exile.

... a man goes with his neighbor into a grove to cut wood; as his hand swings the ax to cut down a tree, the ax-head flies off the handle and strikes the other so that he dies. That man shall flee to one of these cities and live.