Washington (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."

Read More