Shemirat HaLashon שמירת הלשון

We can give yet another reason for King David's attributing eternal life and the good of this world to guarding one's tongue. For it is known that a man's soul is called "the soul of life," for with it a man lives eternal life. But the holy soul of a man who speaks lashon hara departs from him thereby, as stated in the holy Zohar, Parshath Metzora.

And in this world, too, he is destined for afflictions, (the fruits of this bitter sin, as stated in Yerushalmi Peah 1:1), which are exacted of a man for this sin in this world, the principal [of punishment] remaining with him for the world to come.

And the man who muzzles his mouth against speaking forbidden things merits in the life of the world to come, the light secreted for the righteous, as the GRA wrote in the name of the Midrash, that for every moment that a man muzzles his mouth [in this world], he merits [in the world to come] the secreted light, which no angel or [celestial] spirit can conceptualize.

That is why King David of blessed memory exhorted us (Psalms 34:13): "Who is the man who wants life" [in the world to come] and “to see good” [in this world]? “Guard your tongue from evil."

There is yet another obvious reason for the demeaning nature of this bitter sin: It is known that man's great superiority over all living creatures is G-d's having superadded to him a "speaking soul," as it is written (Bereishith 2:7): "And He blew into his nostrils a soul of life and man became a living soul," which Onkelos translates "a speaking soul." And this [(superiority)] is so only if he enhances [the world] with the faculty of speech which the Holy One Blessed be He gave him. Not so, if he employs his speech for evil, in which instance he is worse than the beasts of the earth, which, in any event, cause no harm [by their muteness]. And that is why Scripture emphasizes: "Who is the man who wants life, etc." For otherwise, [i.e., without speech], he is not a man. And this defect [i.e., abuse of speech] is most glaring if he is a Torah scholar, in which instance he is like a great palace with a tannery gutter built into it (as we find in Derech Eretz 1).

And the Gaon, R. Refael, in his book, Marpeh Lashon, explained this verse, too, with great discrimination, on the basis of what is written in Chovoth Halevavoth, Sha'ar Hakniyah, 7: "Many people will come for the day of accounting. And when they are shown their deeds, there will be found in the Book of Merits, mitzvoth which they did not do. And they will say: 'We did not do these!' And they will be answered: 'They were done by one who spoke of you demeaningly [and they were credited to you.]' Likewise, when their [merits] will be lacking from the Book of Merits of those who spoke demeaningly of them, they [the speakers will ask why they are missing], and they will be answered: 'You lost them when you spoke demeaningly of such and such person.' And, likewise, there will be found in the Book of Liabilities of some, acts which they did not commit, and when they say 'We did not commit them,' they will be answered: 'They were added to your account because of such and such that you spoke about, etc.'" And the same has been written in other holy works.

And, this being so, one who is addicted, G-d forbid, to this evil trait of constantly emptying the arrows of his tongue against his friend will not avail his soul with any wisdom or counsel, even if he habituates his tongue always to Torah and mitzvoth. For as soon as he earns a week or two worth of eternal bliss, through Torah and mitzvoth, he will "find" his friend with the arrows of his mouth and give him his merits forever and take in exchange his [the other's] sins. And then, if afterwards he will earn a few months or years more of eternal bliss, he will find a different man and give them to him! And so, ad infinitum, until the day of his death, G-d forbid. In sum, this man is liable to leave the world bereft of his possessions; that is, the eternal acquisitions of Torah and mitzvoth. And he will be "filled" in exchange with many grave sins of infinite variety, from many people, from each in accordance with the shame and humiliation that have been wrought them at his hand in the days of his life. Therefore, King David of blessed memory exhorted us that the beginning of all is to guard our tongues from evil, and then: "Depart from evil and do good." For by guarding his tongue, then as much of the good that the Holy One Blessed be He has accorded him, whether less or more, in any event, will remain in his hand, and his wealth will not be given to others.