In an email dated August 4, 2016, Stone said he "dined" with Assange, the Wall Street Journal said on Monday, citing a person familiar with the matter.
WikiLeaks that summer released a bundle of documents on former Democratic presidential candidate Hillary Clinton, which US officials say was hacked by Russian operatives.
Stone said he was a on a flight out of Los Angeles the day before he sent the email, which sets him thousands of miles away from Assange's location at Ecuador's embassy in London, the report said.
Delta Air Lines confirmed a person named "Roger" was booked on a flight from Los Angeles to Miami on the night of August 3, 2016 but declined to comment if he was on board because of customer privacy rules, according to the Wall Street Journal.