Write a letter informing about your resigning from your post.

Date

To

Dear Sir/Madam,

This is to inform you that with great sorrow I am, hereby tendering my resignation to you. I believe the reasons leading to this decision are well known by you, therefore I shall leave them unsaid at this time.

I would like to express my appreciation for having had the opportunity of being a member of (name of firm) for so many years and offer my best wishes for your continued success.

Thanking you

Yours truly,