How should I answer in HR round if the recruiter asks how can u prove that you are hard working?

I had a project (or assignment, or report) due on second Monday in April. I'd been working on planning throughout March, leaving actual project implementation behind and concentrated more on preparing for XYZ programming contest which was there in the end of March, which I eventually won. When April started, I started working for it.

If the recruiter is asking this question because you said you're hardworking, you're the problem.

Saying you're a hard worker is like saying you're rich - yes it is a good thing but you there're people richer than you out there, and still you brag about it. However you can convey something similar rather than using the words "hard working" by pulling up some recent situation which (story, not you) proves you're a hard worker.

If you're a fresher, here's an example:
I had a project (or assignment, or report) due on second Monday in April. I'd been working on planning throughout March, leaving actual project implementation behind and concentrated more on preparing for XYZ programming contest which was there in the end of March, which I eventually won. When April started, I started working for it. On the first Sunday, my room-mate got badly injured (not a fake story) and I spent next 4-5 days in the hospital. I was left with 5 more days to finish the work. I stayed awake last two days and submitted on time. In the end, I got highest (or top 10 percentile) marks in the class.

And if you are not actually hard working, you don't have stories like these. Any experienced interviewer can catch that easily.