(Do)n't
Qu(it)

Don't quit, do it!

I really like the way it's telling you to do it in multiple ways, it's really unique.

Don't quit, never quit, because you'll never know what was the reward if you do.

Do it, because you could get stuck in a infinite 'I'll do it laters' if you don't.

Do it, and don't quit, because it will be worth it in the end, and you can say you did it, even when others claimed it was impossible.

Unfortunately, I don't know who created this wonderful wordplay quote.