Deuteronomy 19:4-5

This is the case of the man slayer who shall flee there and live. Whoever kills his neighbor unawares, and didn’t hate him in time past;

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