Dear
Heartiest Congratulations on your recent promotion to (position).
I am well aware of how hard you have worked to earn the recognition you presently enjoy at (name of company), and I feel that they are very wise in having made their decision in your favor. I am sure you are enjoying working at this new position and it would have given you a boost to work harder.
Please accept our best wishes for your success in your new position. Keep up the good work. Warm Regards.
Congratulations on Promotion 1