Shemirat HaLashon שמירת הלשון

Therefore, the understanding man must strengthen himself in this [(guarding his tongue)] constantly, and then he will be fortunate in this world and the next. For even with other mitzvoth, if one strengthens himself never to transgress them, his reward is very great, as we wrote above; how much more so with this.

For is it not written (Mishlei 2:4-5): "If you seek for it as silver, if you search for it as hidden treasure — then you will understand the fear of the L-rd." And it is well known that everyone would prefer having a steady income even with less profit than a chance income with greater profit. How much more so with this ["income" of guarding one's tongue], which is a steady income with great profit. For with this mitzvah one can earn both when he is sitting in his house or when he is in the house of prayer or the house of study, and also when he goes to the market and sees people speaking what they should not be speaking (and distancing himself from them.) As Chazal have said (Kiddushin 39b): "If one sat and did not transgress, he is rewarded as the doer of a mitzvah." In sum: With this mitzvah one can "earn" from the time he rises from his sleep [in the morning] until the time he goes to sleep in the evening, effortlessly, and with great profit. For, for every moment of muzzling his mouth he merits the "secreted light," as Chazal have stated.

And [the above is] especially [true] in view of Chazal's statement (Yoma 9b, Gittin 57a) to the effect that the second Temple was destroyed because of the sin of vain hatred and lashon hara. If this sin had the power to destroy what was built, it undoubtedly has the power to prevent the rebuilding of what was destroyed, G-d forbid.

Therefore, the man who strengthens himself in guarding his tongue with all of his power and also spurs others to do so for the honor of the L-rd and His Torah, so that its mitzvoth not be abandoned, G-d forbid, his merit will be very great. For, in truth, the congregation of Israel are in the status of k'sheirim [righteous] and tzaddikim, but [they are remiss in guarding their tongues] because they are deficient in the knowledge of the dinim in this area; and many also lack the counsel to escape the yetzer in this area, all of which we have discussed in the Second Gate. Therefore, if someone were found to awaken them to this, his words would certainly be accepted and the merit of the many would redound to him.

And this is the language of Zohar Chadash in Parshath Lech Lecha: "R. Avahu said: 'Come and see how great is the man who causes another to repent. Whence do we know this? From Bereshith 14:18: "And Malki Tzedek, king of Shalem" R. Chiyya Rabbah taught: 'When the soul of the tzaddik who causes others to repent leaves his body, Michael, the great prince, who presents the souls of tzaddikim before his Creator, goes out and greets that tzaddik, as it is written: "And Malki Tzedek" — this is Michael, the chief of the keepers of the gates of righteousness; "the king of Shalem" — this is the Heavenly Jerusalem. (Ibid.): "brought out bread and wine" — he came forward to him and said: 'Come in peace.'" And in 62b:

"The herald proclaims each day: 'Happy are those who occupy themselves with Torah and those who enable others to learn Torah, and those who "overlook their traits."'"

And this [(abstention from lashon hara)] will certainly help hasten the coming of the Redeemer, speedily, in our days, as this itself furthered the redemption from Egypt, that there was not among them the sin of lashon hara. As we find in Midrash Rabbah, Parshath Emor: "Because of four things Israel was redeemed from Egypt: They did not change their names; they did not change their language; they did not speak lashon hara; and not one of them was found to be licentious." (As to the problem posed by Shemoth 2:14: "In truth, the thing has become known," the commentators have already discussed this.)