Trump may have been referring to very early interviews Ellison gave in 2015, where he said "stranger things have happened" than someone like Trump winning the White House. He compared him to Minnesota's former renegade Gov. Jesse Ventura.
In an ABC News interview in June 2015, Ellison said to George Stephanopoulos: "All I want to say is that anybody -- well, from the Democratic side of the fence, who thinks, who is terrified of the possibility of a President Trump better vote, better get active, better get involved because this man has some momentum and we better be ready for the fact he might be leading the Republican ticket."
Stephanopoulos responded: "I know you don't believe that. But I want to go on."
Ellison said, "We had Jesse Ventura in Minnesota win the governorship. Nobody thought he was going to win. I'm telling you, stranger things have happened."
This morning, in response to Trump's tweet, Ellison responded on the same medium: "My latest call? America is coming together like never before; we'll stop your drive to divide us."