smoking gun 
noun
A piece of incontrovertible incriminating evidence: the trial’s long-awaited smoking gun failed to surface...
- Yes, I think ultimately the White House is going to have to show some concrete evidence, a smoking gun.
- We knew from the Columbia Investigation there were likely to be few smoking guns; the evidence was mostly public testimony and clarified during various news conferences during and after the hearings.
- Each new revelation was the smoking gun that was going to end his presidency.