Negative commandment · Lo Ta’aseh #313 of 613
To not add to the commandments and their understanding
שלא להוסיף על מצוות התורה, בין תורה שבכתב בין בפירושה שקיבלו על פה
The verse · Devarim 13:1
אֵת כׇּל־הַדָּבָר אֲשֶׁר אָנֹכִי מְצַוֶּה אֶתְכֶם אֹתוֹ תִשְׁמְרוּ לַעֲשׂוֹת לֹא־תֹסֵף עָלָיו וְלֹא תִגְרַע מִמֶּנּוּ
“Every matter which I command you, observe to do it: thou shalt not add thereto, nor diminish from it.”
The Rambam’s words
Not to add to the mitzvot of the Torah. [This applies] both to the Written Law and to its explanation conveyed by the oral tradition, as [Deuteronomy 13:1] states: "Carefully observe everything which I command you to do. Do not add to it."
שלא להוסיף על מצוות התורה, בין תורה שבכתב בין בפירושה שקיבלו על פה, שנאמר "את כל הדבר, אשר אנוכי מצווה אתכם--אותו תשמרו, לעשות: .לא תוסף עליו" (דברים יג,א).
Why · Sefer HaChinuch #454
To not add to the commandments and their understanding
“It is from the roots of the commandment that the Master Who commanded us about the Torah, blessed be He, is completely perfect and all of His acts and commands are perfect and good.”
Who & when
“And this prohibition is practiced in every place and at all times by males and females.”
Where in the Mishneh Torah
Hilchot Rebels הלכות ממרים