It's literally the first part of the first sentence of the 12th Amendment to the United States Constitution

The Electors shall meet in their respective states, and vote by ballot for President and Vice-President, one of whom, at least, shall not be an inhabitant of the same state with themselves;

Only one vote cast by an elector may be for an inhabitant of the same state as the elector. If the Floridian elector votes for Trump for President, the elector may not vote for DeSantis for Vice President (or vice versa).