Democratic chairmen of three House committees issued a strongly-worded joint statement Sunday, warning President Trump against possible attempts to obstruct justice.

Trump called in to an interview with Jeanine Pirro on Fox News Saturday night to respond to a New York Times report that the FBI launched an investigation into the President’s relationship with Russia, days after he fired former FBI Director James Comey. While speaking with Pirro, Trump broached the subject of his former lawyer Michael Cohen’s scheduled testimony before Congress.

“[Cohen’s] in trouble on some loans and fraud and taxi cabs… and in order to get his sentence reduced he says ‘I have an idea. I’ll give you some information on the President,'” Trump said. “He should give information maybe on his father-in-law, because that’s the one that people want to look at, because where does that money, that’s the money in the family. And I guess he didn’t want to talk about his father in law.”

Rep. Elijah E. Cummings(D-MD), Rep. Adam Schiff (D-CA) and Rep. Jerrold Nadler (D-NY) issued a joint statement warning the President not to interfere with a congressional witness.

“Our nation’s laws prohibit efforts to discourage, intimidate, or otherwise pressure a witness not to provide testimony to Congress,” the statement reads. “The President should make no statement or take any action to obstruct Congress’ independent oversight and investigative efforts, including by seeking to discourage any witness from testifying in response to a duly authorized request from Congress.”

Cummings is the chairman of the House Committee on Oversight and Reform and Schiff is the chairman of the House Intelligence Committee. Nadler is the Chairman of the House Judiciary Committee.

“The integrity of our process to serve as an independent check on the Executive Branch must be respected by everyone, including the President,” read the joint statement.

Fima Shusterman, Cohen’s father-in-law, loaned at least $20 million to a Chicago cab operator, according to the Chicago Sun-Times. FBI warrants that were used to raid Cohen’s law office and home last April mentioned that operator.

Cohen is scheduled to testify Feb. 7 before the House Oversight Committee. Many expect the hearing to center on elements of Trump’s personal life and business dealings that have come under scrutiny in recent months, including hush-money payments and a proposed Moscow real estate deal.

Cohen was sentenced to three years in prison last month and renounced his loyalty to the President, saying, “I felt it was my duty to cover up his dirty deeds.”

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Write to Alejandro de la Garza at alejandro.delagarza@time.com.