Musar for Deuteronomy 13:1

... And on this the Torah says (Exodus 13:7) "there will not be seen and there will not be found" - it will not be seen b'maaseh (in action) and will not be found b'machshava (in thought), rather one should anul it in his heart. The mitzvot have 3 categories: mitzvot of speech, of the heart and of action, as it is written (Deuteronomy 30:14) "in your mouth and in your heart to do it". Comes the Torah (instructing us) to anul it in the heart, corresponding to the mitzvot which are dependent on the heart. Comes the 'kabbalah' (instructing us) to eradicate it from the house or to burn it, corresponding to the mitzvot of action. And to say 'kol chamira', corresponding to mitzvot of speech. In this way the 3 categories of mitzvot are fulfilled through the prohibition of chametz, teaching you that the prohibition of chametz incorporates all the mitzvot... Just as the 'kabbalah' comes (instructing us) to eradicate chametz and (livdok) to check the house in nooks and in cracks, so too we are obligated to search and check the chambers of our inner being for bad (machshavas) intentions and bad (hirhurim) thoughts. Just as bedikat chametz (checking for chametz) is not valid by sunlight, nor by moonlight, nor by the light of a torch, but only by the light of a candle, so too the bedikah (checking) of the yetzer hara must be by the light of the neshama (soul) which is called 'ner' (candle), this is what is written (Proverbs 20:27) "the candle of Hashem is the soul of man, which searches the chambers of one's inner being.

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."