Trump admitted he fired Comey because of the “Russia thing” in an interview with Lester Holt:

“And, in fact, when I decided to just do it, I said to myself, I said: ‘You know, this Russia thing with Trump and Russia is a made up story, it’s an excuse by the Democrats for having lost an election that they should’ve won.’”

This contradicted the original White House memo that stated Comey was fired because of his handling of the Hillary email investigation. He again backtracked the comments later after claiming that the interview had been “fudged,” though no evidence was offered.

NBC News – Watch Lester Holt’s Extended Interview With President Trump
The Guardian – Donald Trump admits ‘this Russia thing’ part of reasoning for firing Comey
Reuters – Trump seeks to backtrack on 2017 comments on Comey firing