If one has transgressed lashon hora his prayer does not go up in front of G-d because there rests on it a spirit of defilement.

Rava said: With regard to Torah study, at the time when one is engaged in it, it protects and saves...

If one's mouth is perpetually open, and he takes no care [of what he says], then even if he has learned all six orders of the Mishnah and the entire Shas several times, when he comes on high [for judgment], he will find not even one Gemara that will defend him. For each one will be overlaid with the spirit of uncleanliness and will be disgusting to look at.

and Torah study is equal to all of them.

Correspondingly, there are four things that a person has to pay for in this world and the principal remains for him in the future world: Idolatry, incest and adultery, and murder. But slander is equivalent to all of these.

(3) Therefore, I exhort you to train yourself to sit as much as possible, because the sin of the tongue is the most severe, as our Sages said (Tosefta Pe'ah 1): "These are the things...and lashon hara is equivalent to them all." I don't have to elaborate on this most serious sin of all. "All man's toil is for his mouth" (Koheles 6:7).

(6) 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).

(9) R. Chiyya said: "If one speaks lashon hara, all of his limbs are rendered unclean, and he is to be quarantined, because his evil speech ascends and awakens the spirit of uncleanliness upon him, and he becomes unclean. One who comes to become unclean is made unclean [from above]." (As Chazal say in Yoma 39a: "A man makes himself unclean below — he is made unclean from above"). The lower world "triggers" the upper world.

(11) The reason is obvious: Speech is not a specific part of a man as his other limbs are, each one being of benefit in itself. But it is the generality of man and his completeness. And that is why, when he perfects himself by occupying himself with Torah, all of his limbs are amended. For it reposes upon them a spirit of holiness to vivify them and to sustain them in this world and the next. As Chazal have said: "If one's head ails him, let him study Torah, as it is written, etc. If his throat ails him, let him study Torah, etc. If his whole body ails him, let him study Torah, as it is written (Mishlei 4:22): 'And to all his flesh it is healing.'" And the opposite, G-d forbid — by forbidden speech, he harms and defiles all of his limbs, and each and every limb falls from its eminence through this. And what advantage is it to them that their master lives and speaks if only to render din and accounting!

(7) It is written in Yerushalmi Peah 1:1 that just as the reward for Torah study is over and against all of the mitzvoth, so is the punishment for lashon hara over and against all of the transgressions. It is found, then, that just as Torah study is the greatest of all of the mitzvoth, so is lashon hara the severest of all of the sins...

Correspondingly, there are four things that a person has to pay for in this world and the principal remains for him in the future world: Idolatry, incest and adultery, and murder. But slander is equivalent to all of these.

(2) 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...

(3) And in this world, too, he is destined for afflictions... which are exacted of a man for this sin in this world, the principal [of punishment] remaining with him for the world to come.

(6) Also, he is rescued thereby from Gehinnom, as we find in Midrash Tanchuma: "The Holy One Blessed be He said: 'If you wish to rescue yourselves from Gehinnom, distance yourselves from lashon hara and you will merit this world and the next.'"

(1) The Holy One, blessed be He, said to Israel, “Do you want to be delivered from Geihinnom? Keep yourselves far away from the deceitful tongue. Then you will be acquitted in this world and in the world to come.”

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.

(14) There is not a thing to stand in the way of repentance.

MISHNA: Furthermore, for transgressions between a person and God, Yom Kippur atones; however, for transgressions between a person and another, Yom Kippur does not atone until he appeases the other person.

(1) “A man should appease his friend (i.e., a person whom he wronged) on the Eve of Yom Kippur” - Containing four paragraphs.
Yom Kippur does not atone for sins between a man and his comrad (fellow-man) until he conciliates him. Even if he angered him only in words, he is required to appease him (his fellow-man). And if at first he is not pacified, he (must) return and go to him a second and third time. Each time he should take three men with him, and if on the third time he does not become reconciled he (no longer) is obligated to him, (nevertheless afterwards he should say before ten (people) that he did request forgiveness from him), (מרדכי דיומא ומהרי״ל). If he (i.e., the person who was wronged) was his teacher, he must go to him many times until he becomes appeased.
Hagah: The person to forgive should not be cruel by refusing forgiveness (to the one seeking forgiveness), (מהרי״ל), unless his intent is for the good of the one requesting forgiveness, (גמרא דיומא). But if one caused him (the wronged person) a bad name, there is no necessity in forgiving him, (מרדכי וסמ״ק והגה״מ פ״ב מהלכות תשובה ומהרי״ו).

(4) Sins committed against your fellow man are not atoned for on Yom Kippur unless you placate him, as it is said, "You will be cleansed of all your sins before God." This means, only sins against God are atoned for on Yom Kippur, but sins committed against your neighbor are not atoned for on Yom Kippur, unless you placate your neighbor.

(9) And when we reflect but a little upon this awesome thing, we understand immediately why the Holy One Blessed be He is more severe with this sin than with all others, to the point that He refuses to associate with the man of lashon hara at all to rescue him from his troubles, as will be adduced, please G-d, below in the name of the Sifrei, and as is written in the Zohar, Parshath Shelach: "R. Shimon said: 'The Holy One Blessed be He pardoned all, except lashon hara.'"

A man must train himself in this craft of making himself a natural mute all the days of his life, as the GRA has written in Alim Litrufah: "And until the day of his death a man must chastise himself, not with fasts and mortifications, but by putting a rein to his mouth and his lusts. And this is tshuvah [repentance], and this is all the fruit of the world to come, as it is written (Mishlei 6:23): 'For a mitzvah is a lamp; and Torah, light, and the chastisements of mussar [moral discipline], the way of life.' This is [worth] more than all the fasts and mortifications in the world.

(20) What are the best afflictions in the world, which will not disturb me from the service of God? There is nothing more beloved in all of them than that they should curse him, and disgrace him and condemn him... It is better for me to mortify myself with the disgrace of people and their cursing me; and my strength will not depart and I will not be weakened." And with this, when insults come upon him, he will rejoice about them; and just the opposite, he will desire them. And he [should] make a bandage for his heart from these three drugs and train himself with it all of his days.

(11) But the main way to merit Olam Haba is by guarding one's tongue. That is worth more than all the Torah and good deeds... [See also Even Shlomo 7:1 ד"ה ע"י שמירת הפה משתמר הנפש and Even Sheleimah].

Whoever wants to get World to Come cheaply [should follow the advice,] guard your tongue from evil… for guarding one’s tongue is an entry point and a primary means through which one merits a portion in the World to Come...

Whoever guards his speech will merit to be clothed in G-d’s Divine Spirit.

Just as it is forbidden to shame one's friend in matters between man and his Maker, so is it forbidden to shame him in matters between man and his neighbor, even if what is said contains no admixture of falsehood...

One of the sins that people casually trample with their heels [i.e. do not take seriously], is loshon hora.

(16) Have put My words in your mouth

And thus, precisely, is it in our instance. The yetzer bested you today in the area of forbidden speech. Stand up and strengthen yourself against him tomorrow to defeat him. And even if, G-d forbid, he bests you again, notwithstanding this, return and strengthen yourself again against him to wage war with him. And the Holy One Blessed be He will certainly help you, too, to emerge victorious; for "one who comes to purify himself is abetted (from on High.")

Accounting with oneself on the extent of his capability in the service of G-d and to train oneself in it, and be diligent in it, and to hasten and be zealous in doing it until it becomes habit to him, and afterwards to endeavor to do more than what was within one's ability, and to long for it in his heart, and desire it in his thoughts, and to plea to G-d with a faithful and genuine heart and mind to help him and strengthen him for what is above his current ability in understanding (torah) and in good deeds.

When one perseveres more and more in this, the Creator will fulfill his request, and open for him the gates of His understanding, and strengthen his intellect and limbs, level by level, to fulfill the commandments on a higher level, above and beyond his abilities...

Aside from this, the following (Midrash Rabbah 14:4) is well known: "If you have labored much in their [the sages'] words, the Holy One Blessed be He removes the yetzer hara from you" — whereupon I said to myself: "Possibly, if they study this book, which is gathered from all the words of the Rishonim on this subject, and reflect upon them, the yetzer hara for this sin will not prevail so much over them." And, as a mater of course, if one starts to remove himself from this sin, he will come to remove himself from it entirely.

If one wants to be saved from his evil inclination, then he should concentrate all his effort into specifically studying halacha in depth.