I pay attention to what Obama thoughtfully instructs his Press Secretary (Josh Earnest) to say. As of yesterday (via Bloomberg) - Earnest reiterated that Obama has in fact not backed Hillary Clinton.
You know who Obama did come out to publicly support? Joe Biden. It's no secret that he wanted to endorse Biden should he have run - and he made it clear that he thought Biden would be an exceptional President. Since Obama's personal choice - Joe Biden - declined to enter the race, Obama has remained neutral.
You would know that if you stopped reading FiveThirtyEight bedtime stories every night.