Deuteronomy 19:4-6

This is to be the rule for anyone who goes in flight there, after causing the death of his neighbour in error and not through hate; For example, if a man goes into the woods with his neighbour for the purpose of cutting down trees, and when he takes his axe to give a blow to the tree, the head of the axe comes off, and falling on to his neighbour gives him a wound causing his death; then the man may go in flight to one of these towns and be safe: For if not, he who has the right of punishment may go running after the taker of life in the heat of his wrath, and overtake him because the way is long, and give him a death-blow; though it is not right for him to be put to death because he was not moved by hate.