Put One's Money Where One's Mouth Is Meaning

(idiomatic) To take an obvious stake in the truth of a claim that one is making.

Example: To make or take a bet.
2011 September 23, Jason Palmer, “Light speed: Flying into fantasy”[1], BBC News:
  "The scientists are right to be extremely cautious about interpreting these findings," said Jim Al-Khalili, a physicist from the University of Surrey, who suggested that a simple error in the measurement is probably the source of all the fuss..."So let me put my money where my mouth is: if the Cern experiment proves to be correct and neutrinos have broken the speed of light, I will eat my boxer shorts on live TV."