WASHINGTON, DC - JANUARY 22: U.S. President Donald Trump (L) shakes hands with James Comey, director of the Federal Bureau of Investigation (FBI), during an Inaugural Law Enforcement Officers and First Responders Reception in the Blue Room of the White House on January 22, 2017 in Washington, DC. Trump today mocked protesters who gathered for large demonstrations across the U.S. and the world on Saturday to signal discontent with his leadership, but later offered a more conciliatory tone, saying he recognized such marches as a "hallmark of our democracy." (Photo by Andrew Harrer-Pool/Getty Images)
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WASHINGTON, DC - JANUARY 22: U.S. President Donald Trump (L) shakes hands with James Comey, director of the Federal Bureau of Investigation (FBI), during an Inaugural Law Enforcement Officers and First Responders Reception in the Blue Room of the White House on January 22, 2017 in Washington, DC. Trump today mocked protesters who gathered for large demonstrations across the U.S. and the world on Saturday to signal discontent with his leadership, but later offered a more conciliatory tone, saying he recognized such marches as a "hallmark of our democracy." (Photo by Andrew Harrer-Pool/Getty Images)
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WASHINGTON, DC - JUNE 08: Former FBI Director James Comey leaves a closed session with the Senate Intelligence Committee in the Hart Senate Office Building on Capitol Hill June 8, 2017 in Washington, DC. Comey said that President Donald Trump pressured him to drop the FBI's investigation into former National Security Advisor Michael Flynn and demanded Comey's loyalty during the one-on-one meetings he had with president. (Photo by Drew Angerer/Getty Images)
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(CNN) —  

Former FBI Director James Comey wrote in his book that President Donald Trump discussed looking into one of the most infamous allegations in a dossier of claims about Trump and Russia.

In the book, which CNN obtained Thursday, Comey wrote that Trump wanted him to prove the allegation was false in part to prove its falsehood to his wife, first lady Melania Trump.

“He brought up what he called the ‘golden showers thing,’” Comey wrote.

The dossier was commissioned as opposition research by political opponents of then-candidate Trump and compiled by a former British intelligence agent. Portions of the dossier remain unconfirmed.

Comey wrote in the book, “It bothered him if there was ‘even a one percent chance’ his wife, Melania, thought it was true. … He just rolled on, unprompted, explaining why it couldn’t possibly be true, ending by saying he was thinking of asking me to investigate the allegation to prove it was a lie. I said it was up to him.”

Comey recalled the interaction occurred at the same dinner where he said Trump asked for his “loyalty,” which Trump has denied.

CNN reported shortly before Trump took office that top intelligence officials, including Comey, presented Trump with claims of Russian efforts to compromise him. BuzzFeed News published the full, unverified dossier that included the infamous claim that Russian authorities had evidence of Trump watching prostitutes urinate in a hotel suite.

There is no indication that such a tape exists and Trump has vehemently denied it.

Trump lashed out at the report and the publication of the dossier of allegations in a news conference in January 2017, saying, “It’s all fake news. It’s phony stuff. It didn’t happen.”

To bolster his dismissal of the allegations, Trump said, “I’m also very much of a germaphobe. Believe me.”

Comey wrote at length on conversations with Trump about the salacious topic. He wrote that Trump raised the prostitution claim at least four times with him and explained away the allegation in a call with Comey on January 11, 2017.

“‘I’m a germaphobe,’” Comey quoted Trump saying. “‘There’s no way I would let people pee on each other around me. No way.’”

This story has been updated with additional reports.

CNN’s Evan Simko-Bednarski, Greg Krieg, Lauren del Valle and Rob Frehse contributed to this report.