Here is a sample template you can use to write a letter acknowledging that an employee is not fit for work.
To [Recipeint name]
Dear, [Recipient name]
The purpose of this letter is to acknowledge that [Name of employee] is not fit for work for [Number] of days due to [Reason]. We hope you return to work in a healthy and happy state.
We hope you have a speedy recovery and get back to work as soon as possible. The relevant documents regarding medical evidence should be submitted to the office by [Date]. Additionally, if you require extended leave, let us know [Number] prior if possible, and state relevant reasons for this extension.
Take care and get well soon.
[Name of company]