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Dear Sir,
I regret to inform you that I decided to resign from my present position as xxxxxx(ex Sale Manager) with effective from dd/mm/yy.
Thank you for giving me a chance to learn and to gain the valuable experience in Company Name's. I hope my resignation would not cause you much inconvenience.
Thank you for your kind attention and would appreciate if you could let me have a reference letter before I leave.
Yours faithfully,
______________________
your name
dd/mm/yy