The story is that Einstein was shown a German newspaper that claimed "One hundred German physicists claim Einsteins theory of relativity is wrong." Einsteins reply was supposedly, "If I were wrong, it would only take one." I have looked and looked for this story and I can't find it anywhere. Did I just dream of it?
If I were wrong, then one [author] would have been enough!
Einstein’s retort with regard to his theory when he heard that a book titled 100 Authors against Einstein was published in Germany. Quoted in Stephen Hawking, A Brief History of Time (London: Bantam, 1988), 178
To return to this old question with at least a partial answer: there really was a book called "Hundert Autoren gegen Einstein"; you can read it here:
It was published in 1931, two years before the Nazis came to power, and several of the authors, including the one named first on the title page, were in fact Jews. I cannot find a contemporary source for Einstein's reply, but it is surely an example of "se non è vero, è ben trovato".
''When asked to comment on this denunciation of relativity by so many scientists, Einstein replied that to defeat relativity one did not need the word of 100 scientists, just one fact.''
Encyclopedia Britannica Link https://www.britannica.com/biography/Albert-Einstein/Nazi-backlash-and-coming-to-America