President Donald Trump on Thursday lashed out at former Director of National Intelligence James Clapper, who questioned the president's "fitness" for office this week.

"James Clapper, who famously got caught lying to Congress, is now an authority on Donald Trump. Will he show you his beautiful letter to me?" Trump tweeted on Thursday morning.

Trump referred to 2013 testimony in which Clapper denied that U.S. intelligence officials "wittingly" collected data on Americans. Some lawmakers have said he misled them about data collection.

The president also suggests that the ex-intelligence director wrote him a "beautiful letter," but did not say more about when it might've been written or what it says.

After Trump's Phoenix rally Tuesday night in which the president berated media outlets, criticized Republican senators and threatened a government shutdown, Clapper questioned Trump's ability to serve in the office.

"I really question his ability to be — his fitness to be — in this office, and I also am beginning to wonder about his motivation for it," Clapper told CNN on Wednesday.

The president has a history of using Twitter to lash out on public figures who criticize him.