Deuteronomy 19:4-5

4And this is the case of the slayer who shall flee thither that he may live: he that smiteth his neighbour unwittingly, whom he hated not previously;

5as when he goeth into the wood with his neighbour to hew wood, and his hand fetcheth a stroke with the axe to cut down the tree, and the iron slippeth from the handle, and lighteth upon his neighbour, that he die; such an one shall flee unto one of these cities, and live;