Write a letter tendering your resignation.

Dear Sir/Madam,

This is to inform you with deep regret that I am, hereby tendering my resignation to you. Since the reasons leading to this decision are well known to you, therefore, I will not touch upon them again.

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

Sincerely,