Washington (CNN) A communication between President Donald Trump and a foreign leader prompted a whistleblower complaint that is now at the center of a dispute between the director of national intelligence and Congress, a source familiar with the case told CNN.

The concern was first revealed by The Washington Post , which reported that an official in the American intelligence community was so bothered by a "promise" Trump made to a foreign leader that the official filed a formal whistleblower complaint with the inspector general of the intelligence community, citing two former US officials familiar with the matter. CNN has not independently confirmed the detail about a "promise" made to the foreign leader.

The source who spoke to CNN would not disclose the foreign leader with whom Trump was alleged to have spoken.

The Office of the Director of National Intelligence declined to comment. The White House has not responded to CNN's request for comment.

The President dismissed the reporting on Thursday morning, rhetorically asking if there is "anybody dumb enough to believe" it.

"Virtually anytime I speak on the phone to a foreign leader, I understand that there may be many people listening from various U.S. agencies, not to mention those from the other country itself. No problem!," he tweeted.

"Knowing all of this, is anybody dumb enough to believe that I would say something inappropriate with a foreign leader while on such a potentially 'heavily populated' call. I would only do what is right anyway, and only do good for the USA!" Trump said.

The details of the complaint are shrouded in mystery, even as the fight between the House Intelligence Committee and acting director of national intelligence has been brought to the fore by committee Chairman Adam Schiff. While not privy to the details of the complaint, the California Democrat wrote in a letter to the Office of the Director of National Intelligence last week that, from what he has gleaned, he believes "the serious misconduct at issue involves the President of the United States and/or other senior White House or Administration officials."

It's the first time Trump's direct involvement in the complaint has been reported, according to the Post. The complaint was filed on August 12. White House records show Trump had spoken to or interacted with five foreign leaders in the previous five weeks, the Post reports: Russian President Vladimir Putin, North Korean dictator Kim Jong Un, the Prime Minister of Pakistan, the Prime Minister of the Netherlands and the Emir of Qatar. However, it's not clear that the communication that inspired the complaint was with any of those leaders.

A lawyer representing the whistleblower declined to comment to the Post.

The complaint has led to a standoff between Congress and acting Director of National Intelligence Joseph Maguire, who has refused to turn over the complaint to the House Intelligence Committee. Maguire has agreed to testify next week in an open session before the committee after refusing to comply with Tuesday's deadline to hand over the whistleblower complaint , which had been deemed by the intelligence community inspector general to be "credible and urgent."

Schiff announced Wednesday that Maguire will appear at 9 a.m. on September 26. The California Democrat also announced that the intelligence community inspector general will brief the House committee Thursday behind closed doors about how it handled the whistleblower complaint.

On Tuesday, the Office of the Director of National Intelligence sent letters to Schiff and ranking Republican Rep. Devin Nunes of California, saying not only that Maguire was refusing to provide the requested information -- as the complaint "does not meet the definition of 'urgent concern' " -- but also that he would not appear before the committee as scheduled because he "is not available on such short notice."

But by Wednesday, the two sides appeared to have reached a compromise, if only on the timing of the hearing, setting the stage for what could be a contentious public hearing. Maguire will likely be grilled by lawmakers concerned that the administration may have violated whistleblower protections and whether President Donald Trump or top White House officials were involved in the case.

Schiff said Monday that he does not know the exact nature of the complaint, as he has not yet received the details from the Office of the Director of National Intelligence, nor does he know the whistleblower's identity. He has argued that Maguire has taken unprecedented steps to withhold the information from Congress.

According to the Office of the Director of National Intelligence's letter Tuesday to Schiff, obtained by CNN, the complaint does not involve anyone in the intelligence community but rather "stakeholders within the Executive Branch." As a result, its lawyer argues, the complaint is not of "urgent concern" to the office.

The office wrote that it plans to work with the House intelligence Committee but given that executive branch members are involved, there are "confidential and potentially privileged matters" that "will necessarily require appropriate consultations."

In response, Schiff said in a statement Tuesday: "The IC IG determined that the complaint is both credible and urgent, which is why the Committee must move quickly. The Committee's position is clear -- the Acting DNI can either provide the complaint as required under the law, or he will be required to come before the Committee to tell the public why he is not following the clear letter of the law, including whether the White House or the Attorney General are directing him to do so. He has yet to provide the complaint in response to the Committee's subpoena, so I expect him to appear on Thursday, under subpoena if necessary."

While Schiff appeared willing to work with the Office of the Director of National Intelligence on the date of Maguire's hearing, he maintained that the complaint should still be given to Congress.

"The IC IG determined that this complaint is both credible and urgent, and that it should be transmitted to Congress under the clear letter of the law. The committee places the highest importance on the protection of whistleblowers and their complaints to Congress," he said Wednesday.

Schiff is demanding that Maguire turn over the intelligence community inspector general's "determination and all records pertaining to the Office of the Director of National Intelligence's (ODNI) involvement in this matter, including any and all correspondence with other Executive Branch actors such as the White House," according to last week's news release from the congressman's office.

Schiff also argued that Maguire had acted outside the authority of his post by consulting with the Department of Justice about the complaint as he involved "another entity within the Executive Branch in the handling of a whistleblower complaint."

Schiff declined to say whether he has been contacted by the whistleblower or their legal representation, saying he wouldn't want to jeopardize them.

A source familiar with the case told CNN that the Intelligence Community Whistleblowers Protection Act likely offers only one real path forward: circumventing the Office of the Director of National Intelligence and giving the complaint directly to the committee.

That route may also be complicated, the source warned, noting it could raise classification issues tied to transmitting the material.

UPDATE: This story has updated the description about the communication between President Donald Trump and a foreign leader.