How do you write explanation letter for refusing the request of your immediate supervisor to render overtime?

CONFIDENTIAL

To: Mr abc,

Ajhghf

pakistan

Subject: Violating the School Policy

Dear Sir,

It has been reported that you failed to comply the official request from the accounts officer Mr abc on an official matter on Tuesday the 28th May 2013 at 1430 hours where he asked you to come with him for purchasing of some urgently needed items but you refused, where as when Sir abc returned at 1640 hours from the market, he observed that you and the office manager (xyz) had just come back from Saddar.

In view of the above please forward your explanation to the undersigned up to 1000 hours on 3rd June 2013 as to why the disciplinary action should not be taken against you.