Found it. It was written by Rabbi Meir ben Shimon of Narbonne in a letter he wrote against the Kabbalah (published by Adolf Neubauer in JQR 4:3, pg. 358):
"...וכבר הגיע אותו ספר לידינו ומצאנו בו שתלו אותו בר' נחוניא בן הקנה חס ושלום לא היה ולא נברא ולא נכשל בו אותו צדיק..."
Translation: "...And this book has just come into our possession and we discovered that they attribute it to Rabbi Nechunyah ben Hakanah, Heaven forbid. There is no truth to this for never did this righteous individual falter in such a manner..."
By the way, Yisrael Weinshtok wrote an extensive essay on the subject of the authorship of the Bahir in his book במעגלי הנגלה והנסתר (which is how I found the above letter by Rabbi Meir ben Shimon; can be found on Otzar Hachochmah). According to him, when comparing all of the early sources that quote the Bahir, there's good reason to conclude that the book was very likely mistakenly associated with Rabbi Nechunyah. That doesn't mean that it's not a tannaitic work (although at least one Rishon thought it was an amoraic work, and Rabbi Yehuda Aryeh of Modena apparently knew sources that attributed it to the Gaonim), just that it was probably not authored by Rabbi Nechunyah.