Chaim Soloveitchik, ibid., maintains that according to Rambam, ba’al tosif would forbid a posek even from treating a custom or rabbinic injunction with the same stringency as one would a biblical prohibition.
R. Moses ben Nahman (Nahmanides), end of commentary to Deut. 4:2, and R. Hizkiyah ben Manoah, Hizkuni, Genesis 3:3, while not referring to Maimonides explicitly, clearly apply bal tosif to an individual who claims that an action was commanded by God (in the Torah) when it was not. Nahmanides writes [Chavel translation]: “In my opinion, even if someone devised an independent commandment . . . he transgresses . . . . [The prohibition of not adding to the Torah does not forbid] whatever [laws] the sages have established in the way of a fence around the Torah, such as the secondary degrees of forbidden marriages—that activity of [establishing fences] is itself a requirement of the Torah, provided only that one realizes that these [laws] are a result of a particular fence and that they are not expressly from the mouth of the Holy One, Blessed be He, in the Torah.” Cf., however, Nahmanides’ comments on Maimonides’ Sefer haMitsvot, Shoresh Rishon, s.v. “baRishona,” where he states, “First, because it is customary for rabbis to refer to rabbinic matters as ‘Torah’ and strengthen them with biblical citations.” Nevertheless, this may have been done only when it was common knowledge that the ordinances were in fact rabbinic. See also the related comments of R. Joseph B. Soloveitchik, supra.
R. Hizkiyah ben Manoah, Hizkuni, ibid., commenting on Eve’s statement that God forbade even touching the tree of knowledge, cites the Talmud, Sanhedrin 29a, which describes this incident as an example of “he who adds detracts” (kol ha-mosif gore’a). R. Hizkiyah is troubled by the Talmud’s criticism: after all, in what way is Eve’s safeguard any different from subsequent rabbinic gezeirot? He suggests that Eve sinned in falsely attributing the source of the prohibition to the divine. This approach is, of course, in consonance with the view of Maimonides cited above. In fact, R. Joseph Babad, Minhat Hinukh, ibid., actually cites Sanhedrin 29a as proof to the view of Maimonides. R. Reuven Margoliot, Margaliyyot haYam, Sanhedrin 29a, no. 29 also suggests that Sanhedrin 29a supports Maimonides’ position, but cites Avot deRabi Natan, chap. I, sec. 5 which indicates that it was Adam—not Eve—who erred. See also R. Hayyim Joseph David Azulai, Kisei Rahamim, Avot deRav Natan, ibid., perush, s.v. Adam ha-rishon and R. Menahem Kasher, Torah Sheleima, Genesis 3:3, nos. 13-15.
Additional posekim argue that it is forbidden to call a rabbinic edict a biblical prohibition because it violates bal tosif. See: R. Jacob Ibn Forno, ibid.; R. Zvi Hirsch Chajes,