Twenty-fifth Amendment to the United States Constitution
The President may take back the Presidency by sending a letter to the President pro tempore and the Speaker of the House.  However, if the Vice President and the Cabinet think the President is still disabled and still cannot do their job, they can challenge their return.  They have four days to write another declaration saying the President is still unable to do their job.  The Vice President is still Acting President during these four days.  Congress then has to get together within 48 hours, if they are not already in session.  Then Congress has 21 days to make a decision.  In the meantime, the Vice President is still Acting President.