(2) You shall not add anything to what I command you or take anything away from it, but keep the commandments of your God ה׳ that I enjoin upon you.

The Sages taught in a baraita: Forty-eight prophets and seven prophetesses prophesied on behalf of the Jewish people, and they neither subtracted from nor added onto what is written in the Torah, introducing no changes or additions to the mitzvot except for the reading of the Megilla, which they added as an obligation for all future generations.

(1) לא תספו YE SHALL NOT ADD — For instance, to place five chapters in the Tephillin, to employ five species of fruit and plants in the fulfilment of the command of Lulab And to place five fringes on one’s garment. Thus, too, must we explain the following words ולא תגרעו, Ye shall not diminish [from it]" (Sifrei Devarim 82:4).

In my opinion, even if someone devised an independent commandment [rather than altering an existing one] such as establishing a festival in a month which he had devised of his own accord, as Jeroboam did, he transgresses the negative commandment [Ye shall not add unto the word which I command you]. And so did the Rabbis say with reference to the reading of the Scroll of Esther.. However, the prohibition against adding [to the Torah] by word of a prophet we derive only from the verse stating, These are the commandments, which establishes, “From now on, no prophet is permitted to originate anything [in the Torah].” Whatever [laws] the Sages have established in the nature 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.

“Therefore shall you protect [ushmartem] My prized possession [mishmarti], that you do not any of these abominable customs” (Leviticus 18:30). This means: Establish a safeguard [mishmeret] for My prized possession.