Dear (employee name),
This letter serves as your official notice that you have successfully completed your 3-month employee probation as of (effective date). The team at (company name) are delighted with the results that you have produced and your overall performance.
Going forward, we trust that you would continue to put forth your best efforts to achieve the same or even better results. For our part, you can count on us to do everything to provide a great work environment for you.
Now that your initial probation period is complete, please expect annual performance reviews at around this time every year. Thank you for being part of our team, (name).