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.

Article II.  Section 2.  U.S. Constitution adopted 1789.

Please compare and contrast with Compensation of Senators and Representatives, and this post where The Executive announced pay is being raised by decree.