Halakhah su Deuteronomio 18:19

That is that He commanded us to listen to all of the prophets, to do all that he commands - even against a commandment or a group of these commandments. And this is on condition that this be temporary, and that he not command a permanent addition or eradication - as we explained in the introduction of our composition in the Commentary on the Mishnah. And the verse in which the command appears is His saying, "to him shall you listen" (Deuteronomy 18:15). And the language of the Sifrei (Sifrei Devarim 175:2) is, "'To him shall you listen' - even if he tells you to transgress one of the commandments of the Torah [...] temporarily, listen to him." And one who transgresses his word is liable for death at the hands of the Heavens. And that is His, may He be exalted, saying, "And [if] anybody does not listen to the words [of the prophet] who speaks in My name, I will require it of him" (Deuteronomy 18:19). And it has already been made clear in Sanhedrin (Sanhedrin 89a), "[In the case of] three, their execution is at the hands of the Heavens: One who transgresses the statement of a prophet, a prophet who violated his own statement and one who suppresses his prophecy." It is all from His saying, "does not listen (yishma) to the words [of the prophet] who speaks." Read into it, "Who shall not voice (yashmia)." And the regulations of this commandment have already been explained at the end of Sanhedrin. (See Parashat Shoftim; Mishneh Torah, Foundations of the Torah 7.)

To not marry one of all the sexual prohibitions: To not indulge [we not indulge] in one of all of the [women forbidden by] sexual prohibitions - and they are the [close] relatives, a married woman and a menstruant - and even without intercourse, such as hugging and kissing and all that is similar to these evil acts that licentious ones, 'that go after vanity and become vanity,' develop expertise about. As it is stated (Leviticus 18:6), "Any man shall not approach any of his own flesh, to reveal nakedness" - and its understanding is as if it stated, "Do not do any approaching, which is what causes and brings a person to reveal nakedness." And so did they, may their memory be blessed expound (Sifra, Achrei Mot, Chapter 13:15, 21), "'Shall not approach to reveal' - I only have not to reveal. From where [do I know] not to approach? [Hence] we learn to say, 'And to a woman in the impurity of her menstruation you shall not approach' (Deuteronomy 18:19). I only have a menstruant with 'do not [approach' and 'do not] reveal.' From where [do I know] for all sexual prohibitions? [Hence] we learn to say, 'do not approach to reveal.'" And there it is said, "'And the souls that do, will be cut off' (Deuteronomy 18:29) - lest you say they will be liable for excision with approaching alone. [Hence] we learn to say, 'that do,' and not 'that approach.'"

And this commandment is practiced by males and females at any time that a prophet is found among us. And one who transgresses it and does not heed him is liable for death by the hand of the Heavens, and as it is written (Deuteronomy 18:19), "And it shall be that the man who does not heed the words, etc. I will require it of him." And our Sages, may their memory be blessed, said in Sanhedrin 89a, "The death of three is by the hand of the Heavens: one who transgressed the words of the prophet; the prophet who transgresses his own words; and one who suppresses his prophecy (does not say it)." And all of this is in the understanding of the verse, as it stated "who does not heed (yishma), etc." - and they, may their memory be blessed, said, "Read in it, 'not heed (yishma),' and read in it, 'not make heard (yashmia).'"