In yesterday's Seforim post by Marc Shapiro writes the following:
[Here is an] "announcement by Agudas ha-Rabbonim, which appeared in Ha-Pardes, November 1975.
"Yet the beautiful thought which the learned rabbis assume was stated by Hazal was actually stated by Ahad ha-Am, and is perhaps his most famous saying. However, for one whose only Jewish knowledge comes from the yeshiva, this information would be unknown, and it is easy to see how such a statement (“more than the Jews have kept the Sabbath the Sabbath has kept the Jews”) could “infiltrate” this closed world and become just another ma’amar hazal."
While this is not the same thing as a statement by the Agudas Ha-rabbonim calling a ma'amar Ahad Ha'am a ma'amar Hazal, I recall a few years ago a rabbi quoted this aphorism (without attribution to Hazal) in the Country Yossi Family Magazine. A reader sent a letter saying what the source was and that therefore the statement was wrong. The rabbi responded that whoever said it, it's true!