I don't know how reliable this site is, but the official Xbox FB page did share one of their articles, so maybe it's legit. I just didn't think they'd be working on their next systems this early.

It really is amazing how fast console technology is moving along these days. It seemed like just yesterday that the PlayStation 4 the Xbox One were introduced on the market. Soon, both will be outmoded by their successors. Both Sony and Microsoft respectively have confirmed the upcoming release of PlayStation 5 and Xbox Two. Right now, we do not have a lot of detailed information about either, but we can make some predictions about what we can expect from both.

Source

The PS4 and XBO aren't even that old yet.