The letter you're referring to was written by Colonel Lewis Nicola on May 22, 1782, while George Washington was camped at Newburgh, New York. Nicola, troubled by the financial instability and dysfunction of the Continental Congress, proposed that Washington consider becoming king of the United States under a constitutional monarchy.
Washington’s reply was swift and unmistakably firm. He wrote:
“Be assured, Sir, no occurrence in the course of the War, has given me more painful sensations than your information of there being such ideas existing in the Army as you have expressed... If I am not deceived in the knowledge of myself, you could not have found a person to whom your schemes are more disagreeable.”