The Twenty-fifth Amendment Section 4
Whenever a Vice President and a majority of the principal officers of the executive departments OR other body that Congress may by law provide, transmits to the President pro tempore of the Senate and the Speaker of the House, their written declaration that the President is unable to discharge the powers and duties of his office, the Vice President immediately becomes Acting President.
The President may transmit his written declaration than no inability exists.
HOWEVER, if the Vice President and a majority of principal officers of the executive department or such other body as Congress provides, transmit within four days their written declaration that the President is unable to discharge the powers and duties of this office -
THEN, Congress must meet within 48 hours and decide the issue within 21 days after meeting. They decide by two-thirds vote of both Houses.