I suggest using a hyphen to avoid any confusion. Without any punctuation between two adjectives, it is left to the reader to interpret whether you mean, "hard, earned money" or "hard-earned money". The two are very different.
If it is "hard-earned money" you mean, I find it easier to simply insert the hyphen so you say what you mean instead of relying on the fact that it is a common phrase. That way you'll cultivate good habits for when you need to write some not-so-common phrases.