《管子·版法解》
因为使用民力过头，事情就不会做得完善。不完善而不断返工，所以叫作 劳 。
用力苦则事不工，事不工而数复之，故曰劳矣。
Because if you overuse the people, the task will not be done thoroughly. And if a task is not done thoroughly, people will have to rework and so become weary.