The New Yorker broke the story deep into a long, long article ("Donald Trump's New World Order") that in January, President Trump called out Palestinian Authority chairman Mahmoud Abbas for his public disparagement of Jewish history:

[A]t a meeting of the Palestinian National Council in late April, in Ramallah, he declared that Ashkenazi Jews came not from the Biblical holy lands but from the Turkic empire of Khazaria, and that the Nazi slaughter of European Jews was the result not of anti-Semitism but of their financial activities – "usury and banking and such."

This is a discredited (by DNA!) attempt to claim that Jews have no right to Israel because they are imposters posing as the people of the Bible.  It enjoyed some popularity among anti-Semites for a number of years, but it has faded thanks to scientific refutation – except, apparently, among the Palestinian leadership.

Trump's response:

On January 17th, the New York Post published a column by Michael Goodwin, a Trump partisan, with the headline "abbas' jew hatred exposed."  The column described a speech in which Abbas had made comments disparaging Jewish history.  It featured a photograph of Abbas waving two clenched fists.  On a copy of the article, Trump wrote a note in large black script, "Mahmoud, Wow – This is the real you?"  He signed it, "Best Wishes, Donald Trump."  Some of his aides argued that it would be undiplomatic to send the message.  Kushner loved it.  "That was the President being the President," he told aides.  The White House sent Trump's message to Donald Blome, the consul-general in Jerusalem, who had it delivered to Abbas at his headquarters in Ramallah.  Kushner told aides that Trump was challenging Abbas, saying, in effect, "I want to know, are you a great leader or are you a terrorist?  You show me.  It's your choice."

Here is a photo of the message that has appeared in a few places.