Does Trump have a taping system in place? Its (almost) certainly a bluff, a way to humiliate Comey on his way out the door and presumably to discredit him
Let's apply a bit of logic to this, shall we?
If Comey didn't say that to Trump, he would surely know it, right? Given that obvious truth, how could Trump possibly bluff him? Comey could testify under oath that he didn't say it, and if he really didn't say it there would be no tape to refute his testimony and Trump would be proven wrong.