put up or shut up

(idiomatic) To desist from saying something unless one is able to prove it; to act in a manner that makes further talk unnecessary.

Example: 2005 June 21, Cornelia Dean, "Opting Out in the Debate on Evolution ," New York Times (retrieved 22 June 2011):
  "If the evidence for modern Darwinian theory is so overwhelming, they should have called the bluff on the other side and come and made their arguments. . . . They should have put up or shut up."