Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [name of employee],
This letter certifies that you've finished your three-month probation, effective [date].
All of us here at [name of company] are tremendously pleased with your performance to date, and we expect nothing but the best efforts from here on in. In turn we'll do everything we can to provide you with a healthy, rewarding work environment.
Now that your three-month probation is finished, you can expect an annual review at this time each year.
All the best,