President Donald Trump on Thursday expressed surprise that his eldest son, Donald Trump Jr., has been subpoenaed by the GOP-controlled Senate Intelligence Committee.

"I was very surprised," Trump told reporters at the White House, adding "we'll see what happens" when asked if his son should fight the subpoena.

"My son is a very good person, he works very hard," Trump said. "The last thing he needs is Washington, D.C."

The president also recounted how Trump Jr. approached him before Trump's presidential run. Trump said his son told him politics was "not my expertise" and "not something I really like," but that he would offer assistance because "you're my father."

Trump then pointed to special counsel Robert Mueller's 400-plus page report, saying "that's the bible."

"The Mueller report came out," he said. "It said he did nothing wrong."

Mueller declined to charge Trump Jr. and other senior aides involved in a controversial June 2016 Trump Tower meeting he helped arrange involving a Kremlin-connected lawyer who promised but didn't deliver on damaging information on 2016 Democratic presidential nominee Hillary Clinton.

The special counsel said in his report that he declined to charge Trump Jr. with campaign finance violations in part because it would be difficult to prove the participants knew their conduct was unlawful.

The Senate Intelligence Committee subpoenaed Trump Jr. — the first known congressional subpoena of a member of Trump's immediate family — to answer questions about his contention that he only had limited knowledge of the proposed Trump Tower Moscow project, a source with direct knowledge of the matter told NBC News. The news was first reported by Axios on Wednesday.

Two sources familiar with the committee's subpoena told NBC News it was served in in mid-April. Those sources would not say when or if there is a deadline for Trump Jr. to respond.

Trump's eldest son testified before the Senate Judiciary Committee in September 2017, saying at the time that he was only "peripherally aware" of the Russian real estate proposal, which had not been revealed to the public during the campaign.

Trump's former longtime attorney and fixer, Michael Cohen, testified to lawmakers, however, that he briefed Trump Jr. and his sister Ivanka Trump on the project "approximately 10" times.

Cohen is now serving a three-year prison sentence for a variety of federal crimes, including lying to Congress in earlier testimony about the timing of the project when he said it was abandoned in January 2016, before the heart of the Republican primary campaign.

In fact, talks on the project continued until June 2016, even as Trump was telling the public he had no business dealings with Russians. Cohen said he told the lie in order to stay in line with Trump's preferred narrative.

The Intelligence Committee also wants to ask Trump Jr. additional questions about that controversial Trump Tower meeting. Trump Jr. had told the Judiciary Committee that he did not inform his brother-in-law Jared Kushner and then-campaign chairman Paul Manafort — who both attended the meeting — what it was about, or tell his father about the meeting at all.

A source close to Trump Jr. told NBC News in a statement that the president's son is "a private citizen who has already been cleared by Mueller after a two-year investigation." The source noted Trump Jr.'s more than 30 hours of testimony before various congressional committees.

Since the subpoena was reported publicly, many Republicans and allies of the president and his son took aim at Senate Intelligence Committee Chairman Richard Burr, R-N.C.

"Don continues to cooperate by producing documents and is willing to answer written questions, but no lawyer would ever agree to allow their client to participate in what is an obvious PR stunt from a so-called ‘Republican’ Senator too cowardly to stand up to his boss Mark Warner and the rest of the resistance Democrats on the committee," the source continued.