Article II, Section I of the U.S. Constitution:
The President shall, at stated Times, receive for his Services, a Compensation, which shall neither be increased nor diminished during the Period for which he shall have been elected, and he shall not receive within that Period any other Emolument from the United States, or any of them. Before he enter on the Execution of his Office, he shall take the following Oath or Affirmation: —“I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my Ability, preserve, protect and defend the Constitution of the United States.”
Good lord! I never knew this is what the presidential oath of office means:
