From The Washington Post
The WikiLeaks email to which Trump was referring was a March 2015 exchange forwarded to Clinton attorney Cheryl Mills from Nick Merrill, who now acts as Clinton’s press secretary. Several Clinton aides were discussing how Obama had told the news media that he learned about Clinton’s private server from news reports. Mills responds, “we need to clean this up — he has emails from her — they do not say state.gov.” In other words, Obama had emails from Clinton’s private server — suggesting that he should have known or did know about her server before the public.
“That’s why he stuck up for Hillary,” Trump told Reuters’s Steve Holland and Emily Stephenson, “because he didn’t want to be dragged in. Because he knew all about her private server. This means that he has to be investigated.”