Musar zu Dewarim 13:1

And now we will speak about the matter of punishment for the nullification of positive commandments: Our Rabbis, may their memory be blessed, said (Ketuvot 86b) [that] if [the court warned] someone to perform [the commandment of the] sukkah or [of the] palm branch (lulav) “and he does not do so, we strike, etc.” And they said (Rosh Hashanah 17a) that men who have never placed tefillin on their heads are called “rebellious Jews with their bodies,” and their punishment is more severe than one who transgresses once against a sin for which he is liable for excision. And they said [that] all whose sins are greater than his merits and among [his sins] is the sin of rebellious Jews with their bodies - for example, one who has never worn tefillin or one who was involved in transgressions such as forbidden sexual relations - descend and are judged in Gehinnom for twelve months. After twelve months, their body is finished; and their soul is burnt; and the wind spreads it under the soles of the feet of the righteous ones, as it is stated (Malachi 3:21), “And you shall trample the wicked to a pulp, for they shall be dust, etc.” And they said (Sanhedrin 99a), [that] one who is permissive (in his eyes) [regarding] positive commandments, such as one who belittles the intermediate days of the festival - which is from a positive commandment, as it is stated (Exodus 23:15), “You shall observe the Feast of Matsot” - has no portion in the world to come, even if possesses Torah and good deeds. And there is a general warning of a negative commandment, for all the positive commandments - as it is stated (Deuteronomy 13:1), “neither add to it nor take away from it.”

Commandments 454 and 455 warn us neither to add nor to subtract from the commandments. The Torah (13,1) writes: "Neither add to it nor take away from it."

And the commandment, which is the interpretation of the Torah, he did not write down, but commanded it to the Elders and to Joshua and to the rest of Israel. As it is said, "All this word which I command you, that shall ye observe to do : thou shalt not add thereto, nor diminish from it" (Deut. 13:1). And because of this it is called the Oral Law. And from the days of Moses, our teacher, until our sainted Rabbi, Rabbi Judah the Prince, the Oral Law was not written in a book from which the multitude could study, but they studied everything orally, and everyone would take notes for his own use on the things he learned from his teachers, and would teach orally to the many. And so everyone would write for himself, according to his ability, the explanation of the Torah and its laws just as he heard it; and those things which were created anew in every generation, and laws that they had not learned from tradition but through the application of the "thirteen rules of interpreting the Torah," and on which the great Bet Din was in agreement. And so did they do until the matter came before Rabbi Judah, the Prince. And when Rabbi Judah saw that in his days the situation had altered, for he saw that the pupils were growing less and less, and that new troubles were coming upon the Jews and that the dominion of Rome was spreading over the world and growing mightier, and that Israel was wandering to the borders of distant countries only to be enslaved; when he saw this state of affairs, that Israel was confused and that the Torah was being forgotten from Israel, then he compiled the Mishnayot so that they could be in the hand of every single person so that they could learn them and they would not be forgotten. And he sat all of his days, he and his colleagues, and they taught the Mishnah to the multitude. And there were in his presence the greatest of the wise men of Israel and they received the Mishnah from him, and with them were thousands and thousand of other Sages. And the pupils of Rabbi Judah, the Prince, likewise compiled their own compilations. Rav compiled the Sifra and the Sifré to explain and make known the principles of the Mishnah. And Rabbi Hiyya compiled the Tosefta to explain points of the Mishnah. And Rabbi Hosaya and Bar Kappara compiled the Baraitot to explain the words of the Mishnah. And matters developed in this manner until the Amoraic came and they had differences of opinion as to the meaning of the Mishnayot and the Toseftot and the Baraitot until Rabbi Johanan compiled the Jerusalem Talmud in the land of Israel almost three hundred years after the destruction of the Second Temple, and it was called the Jerusalem Talmud because Rabbi Johanan lived in Jerusalem. About a hundred years later, Rav Ashi, who lived in Babylonia, compiled the Babylonian Talmud. And the purpose of both of these Talmuds is to explain the words of the Mishnah and to interpret its deep meaning, and the things that were created anew with the coming of every Bet Din from the days of our sainted rabbi, Rabbi Judah Hanasi, until the compilation of the Gemara. And from the two Talmuds and the Tosefta, and from the Sifra, and the Sifré and the Baraitot, from all of them we can find the explanation of what is forbidden and what is permitted, what is clean and what is unclean, what is obligatory upon us and what we are exempt from, what is unfit according to the Torah and what is fit just as they copied it, one man from another's mouth, all the way back to the mouth of Moses, our teacher, of blessed memory, as he received it from Sinai.