Musar על סנהדרין 103:4

I also implore and plead with you to guide your daughters very carefully to refrain from cursing, swearing, lying and fighting. Rather, everything they do should be done peacefully, with love, affection and gentleness. I have left behind several Yiddish books on Mussar (morality). See that [the children] read them constantly, especially on the Holy Shabbos, when Mussar is the only thing they should read. Always instruct them according to Mussar books. Don't hold back from hitting them when they curse, swear or lie. Don't be lenient with them, because parents will be punished severely for the corruption of their children, G-d forbid. And even if one constantly teaches them Mussar, but they do not follow it, one's sorrow and shame in the World-to-Come will be great. As it is written (Vayikra 21:9), "She defiles her father" - [in such a case] the son of a righteous man is called "the son of a wicked man" (Sanhedrin 52a). Similarly, in other matters, lashon hara and gossip. Their eating and drinking should always be preceded and followed by the appropriate blessings. They must be careful to say the blessings, Birkas Hamazon and Krias Shema with proper kavanah (intent). Most importantly, they must not wander outside the home and must obey and respect you and my mother and all their elders. They also need to observe all that is written in the Mussar books.

באש ישרופו אותו ואתהן . When someone marries a wife [mother-in-law. Ed.] and her mother (20,14), the Torah decrees death by "burning" for all three parties. Our sages teach us the type of "burning" the Torah has in mind for this transgression as well as for all other instances where death by "burning" is decreed. It is similar to what happened to Nadav and Avihu whose souls were burned while their bodies remained lifeless but intact. If the Torah would have written ישרפו באש, the fire would have first consumed the body as is the case when someone is "burned at the stake" Since the Torah mentions the word אש first, it is clear that a different kind of burning is meant.

From all of our elaborating until now on the greatness of the punishment for lashon hara in this world and the next, it should be understood how much we must distance ourselves from the sin of machloketh. For aside from the sin itself, which is a grave one, as we shall explain, it is a potent cause of other grave sins, namely: vain hatred, lashon hara, rechiluth, anger, verbal wronging, "whitening of the face" [in shame], revenge, bearing a grudge, vain curses, undermining a person's livelihood, and, sometimes, even desecration of the Name, G-d forbid, an extremely grave transgression. And it is also common to come through this to the sin of flattery, whereby one gains adherents for his quarrel, as we find with Korach, as Chazal have said (Sanhedrin 52a) on the verse (Psalms 35:16): "Because of the flattery of the quaffing of a draught, he has ground his teeth against me" — Because of the flattery they accorded Korach for the drinks that he plied them with, the plenipotentiary of Gehinnom ground his teeth against them." And machloketh also leads to levity, to mock the opposing party and thereby draw adherents to one's counsel. And all this was the conduct of the first man of machloketh — Korach, as we find in Midrash Rabbah, Korach. And it is known that the punishment for levity begins with suffering and ends with destruction, as Chazal have said. And more than this. It is found that when the yetzer entices one to strengthen the machloketh and to draw men to his counsel, and he fears lest they turn away from him and leave him by himself, the yetzer entices him to create a strong bond [of unity] by means of an oath. All this we find in the Midrash and in the Gemara in respect to Korach, Dathan, and Aviram. And see, my brother, how much blindness there is in this. For the oath is close to being a vain one, their having sworn to transgress a mitzvah [See Yoreh Deah 236:2, and the Shach there, section 4]. And even if he fulfills his oath, it still does not leave the category of "a vain oath" [See 238:5]. And the severity of the punishments for a vain oath is well known, the Holy One Blessed be He not absolving one for this, as it is written (Shemoth 20:7): "The L-rd will not absolve one who takes His name in vain" [See Shevuoth 39a]. I have written all of this only to show to all the great "blindness of the eyes" that inheres in this. And even if in the beginning he does not intend such evil, still, in the end, he will not be absolved of the aforementioned transgressions, as should be clear to anyone with a knowledge of the world.