Dear Mr.
Thank you very much for considering me for the position of QA Developer and for inviting me to interview. I sincerely appreciate the opportunity to interview.
However, it is with regret that I would like to decline your interview offer because I have accepted a job with another organization.
I wish you all continued success and I hope our paths will cross again in the future. Thank you once again for your time and consideration.
Regards,
Apisada Feanma