Menachos 30a - Torah Min HaShamayim

The gemara quotes a machlokes whether the last eight pesukim in the sefer torah (from וימת שם משה עבד השם), were written by Yehoshua, or whether Moshe wrote them "in tears" [rashi explains the idea of writing in tears to mean that Moshe normally would repeat back the Torah as he transcribed it to avoid errors, but the last eight pesukim he was too distressed to repeat it and therefore wrote it without repeating it back]. The gemara concludes that this machlokes has very little bearing an anything we do practically because even the opinion who says that it was written by Moshe considers it to be distinct from the rest of the Torah and that it should be read by one person. Therefore, it would seem irrelevant to say that we "pasken" like one opinion rather than the other since there is not halachic debate between them. However, Rabbeinu Bachya (on the pasuk of moshe dying) argues on the Ibn Ezra who says that it was written by Yehoshua. Rabbeinu Bachya writes -

ואינו אמת, ואין פירושו נכון, אבל הנכון להאמין והקבלה האמתית שיש לנו, כי משה כתב התורה כולה מבראשית עד לעיני כל ישראל הכל מפי הגבורה, והנה משה כמעתיק מספר קדמון מתחלה ועד סוף אות באות, וזו היא דעת רבי מאיר (דלא כגירסא שלנו) שאמר אפשר ס"ת חסר אות אחת וכו' ונ"ל שאין לתמוה אם יכתוב משה "וימת שם משה וכו' שהודיע בזה מה שעתיד להיות וכו' שכל הנביאים נוהגים המנהג הזה בדבריהם לדבר עבר במקום עתיד

It seems strange for Rabbeinu Bachya to argue strongly about something which is an earlier Tamudic debate. Clearly, Rabbeiu Bachya understands that the "halacha" is like the latter opinion that we must believe that Moshe wrote the entire Torah beginning to end. Perhaps he understands this from the fact that the gemara tries to fit the statement of Rav with R. Shimon who says that Moshe wrote even the last eight pesukim. Nonetheless, Rabbeinu Bachya considers this opinion to be the prevailing opinion and calls it the קבלה האמתית - the true tradition.

A few months ago I gave a shiur on the topic of Torah Min HaShamayim where I discussed the yesod of the Rambam and what exactly we are obligated to believe - download here. Rav Moshe has a Teshuva where he considers an old sefer to be a forgery due to it's claim that parts of the Torah were inserted after Moshe Rabbeinu. Rav Moshe considers it an unequivocal violation of Torah min HaShamayim to claim that even a single letter in the Torah was written by somoene other than Moshe Rabbeinu (with the exception of the last 8 pesukim which is the discussion in the Talmud).