Is there a saying like that? I'm thinking it's something along the lines of working something too hard. I don't want to attempt to word it because I'll most likely butcher it horribly.
I believe the expression you're looking for is "never spur a willing horse". The meaning, if it's not obvious, is that you don't need to tell someone to do something if they're already doing it.
This idiom exists in several languages and goes back at least a few centuries. I found several variations in Proverbs, Maxims and Phrases of All Ages (Christy 1887).
The other possibility is beating a dead horse where continuing a discussion is a waste of time.