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There is a negative mitzvah not to add onto the mitzvot of the Torah as it says in the pasuk "אֵת כָּל־הַדָּבָר אֲשֶׁר אָנֹכִי מְצַוֶּה אֶתְכֶם אֹתוֹ תִשְׁמְרוּ לַעֲשׂוֹת לֹא־תֹסֵף עָלָיו וְלֹא תִגְרַע מִמֶּנּוּ". 
Nature of the Prohibition
- Some say that the prohibition means that one may not permanently establish a new mitzvah. Some say that it a Bet Din declares that a rabbinic prohibition is actually a Biblical one they are in violation of Baal Tosif. Others disagree.
- Some say that the prohibition includes fulfilling a mitzvah in an expanded or enlarged manner.
- Some say that there's no prohibition of Baal Tosif to repeat a mitzvah several times. Some disagree.
- Some say that saying Shema at night before its time with intent to fulfill the mitzvah is Baal Tosif while others argue that there can't be Baal Tosif to say Shema since it is learning Torah.
- The Sephardic minhag to say Sefirat Haomer in the morning besides the night is an example of this.
- Saying Kedusha multiple times a day isn't Baal Tosif.
- There is no issue of Baal Tosif when one's intention is to follow the Rabbis such as by keeping a second day of Yom Tov.
- If it isn't the time of the mitzvah there's no way to violate Baal Tosif unless one has intention to fulfill the mitzvah then. For example, sleeping or eating in the Sukkah before Sukkot isn't an issue of Baal Tosif if one doesn't have intention to fulfill the mitzvah. After Sukkot is considered the time of the mitzvah and is a problem to sit in the Sukkah immediately after Sukkot even without intention.
- Devarim 13:1, Rambam (Sefer HaMtizvot Lavin n. 313), Sefer HaChinuch 454
- Ramban Devarim 4:2, Rambam (Mamrim 2:9)
- Rambam Mamrim 2:9
- Raavad Mamrim 2:9
- See examples below and Encyclopedia Talmudit "Bal Tosif"
- Sifrei Devarim 13:1
- Rosh Hashana 28b
- Tosfot Rosh Hashana 16b says it in reference to blowing extra shofar blowings on Rosh Hashana or picking up the Lulav several times in a day.
- Turei Even Rosh Hashana 16b
- Igrot Moshe 2:60 s.v. umah writes the Mishna Brurah implies that there's Baal Tosif if one says Shema too early with intent to fulfill the mitzvah therefore one should have intent not to fulfill the mitzvah. Rav Moshe argues that Shema is essentially a piece of Torah and there's prohibition to relearn Torah multiple times. See Chavot Daat Bet Hasafek Safek Safeka n. 20 who writes that saying Shema twice even within the obligated time is prohibited.
- Yalkut Yosef 489:25. See however, Teshuvot Vehanhagot 2:247 who says it is Baal Tosif.
- Rashba Teshuva 1:249, Rama O.C. 125:2
- Igrot Moshe 2:60 s.v. umah
- Chazon Ovadia Sukkot p. 481 citing Shalmah Yosef 12, Shem Chadash 22b