If you would like to congratulate someone for clearing an interview, here is a sample template you can use to write an acknowledgment letter after an interview.
This letter is in reference to your successful completion of the interview round dated [Date] for the post of [Job Role]. We congratulate you on making it this far, it clearly exhibits your potent skills and preparedness.
We hope that you learned something valuable in this interview experience with our diligent and expert panel. Also, we wish that you utilize this experience to work harder on honing your skills and overcoming your shortcomings.
The results will be announced shortly, until then, your patience is requested. Thank you for your participation, we wish you good luck for the results.