Writing Letter For Serving Notice Period

Dear Esteemed Members,
I am working as HR Executive in a company.
One of my colleague has put his resignation and is asking to relieve him within 10 days. But as per the appointment letter issued to him he has to serve 30 days notice period.
Now I need to write a letter to him asking him to serve the notice period and then only we can relieve him as we working on important assignment and cant leave it half done.

Your approach seems to be right. But what is your question/ query here?

Simply write a mail
Dear XYZ,
Refer to your appointment letter regarding Resignation and Notice Period.You are required to serve notice period of 30/45/60 days and complete the projects assigned.
As you know that the project on which you are working on need your time as it can not be transferred to a junior employee.So kindly complete the pending assignment so that after completion we can process with your exit formalities.
Signatures
Hope it will be helpful.

Swati,
Normally In lieu of notice period salary for the notice period is stipulated. Is it there in the appointment letter? If it is there ask him to deposit one month's gross salary amount. if it is not there, if you decide to accept his resignation, ask him to serve one month further from the date of receiving application.