old tale: In the English fairy tale known as "Mr. Fox," a murderous wooer, discovered in his crimes by the lady he seeks to marry and victimize, repeatedly disclaims her story of what she has seen by saying, "It is not so, nor it was not so. And God forbid it should be so."
Benedick uses the same words to mock Claudio's reluctance to admit his "crime" of falling in love.