I'd rather say that working hard will only make your boss richer, not yourself. On the other hand, there is an age-old adage which says that you should be working smarter, not harder.

indeed! I agree with this idea. We need to work smart rather than work hard because if you work smart your career grow fast and you will be notice easily by your employer so career move on your part and it can save time, your employer offered you a higher position compare to work harder your doing the traditional way to achieve your output or quantity.