I've just googled and it was reported in The Daily Beast, then repeated in Yahoo News using The Daily Beast as their source.
The Daily Beast's source is an 'anon former staffer' who states 'It is absolutely true' and it's written in the style of a Take a Break story.
Then a couple of other publications ran with it. The Royal Observer believes it's due to 3 areas; Harry's potential return/reunion, Charles' schedule/health, and that Charles needs to modernise the monarchy.
The only thing any of them can say with certainty is that they were all at Balmoral at the same time.