Musar su Salmi 12:5

And this is the thing about the group of slanderers:
Our Rabbis, may their memory be blessed, said (Arakhin 15b), “Anyone who speaks evil speech is as though he denied a fundamental principle [of faith] - as it is stated (Psalms 12:5), ‘Who have said, “we will make our tongue mighty; our lips are with us; who is lord over us?”’” Therefore, they considered him as if he denied a fundamental principle. For he causes and brings about great damage and much evil to his fellows by making them foul in the eyes of people or by other ways of [causing them] loss. And it is not likely that a man would prepare a mechanism of destruction and damage that is more bitter than death without it benefitting himself monetarily, unless it is to have his [evil] impulse honor himself and remove the yoke of the Heavens from upon him - to remove ethical constraints - as it is stated in the Psalm, “when the Ziphites came and told Saul, etc., O God, arrogant men have risen against me; a band of ruthless men seek my life; they are not mindful of You.” (This is a scribal error, as there is some difference between the verse brought down by the author, may his memory be blessed, [and his source]. As [in the source], it is written, “For strangers have risen against me, etc.” And the verse that he wrote here is found in Psalms 86:14; but it needs to be corrected and replaced with the verse as it is written there in Psalms 54:5.) And our Rabbis, may their memory be blessed, explained (Midrash Tehillim 54) that they intended that Saul would bless them, as he said to them (I Samuel 23:21), “May you be blessed of the Lord for the compassion you have shown me!” But they did not place God in front of them - as it is written in His Torah (Deuteronomy 27:24), “Cursed be he who strikes down his neighbor in secret.” And it is written (Ecclesiastes 10:11), “no advantage is gained by the master of the tongue!” And it is stated about Doeg (Psalms 52:4-5), “Your tongue devises mischief… You prefer evil to good; the lie, to speaking truthfully. Selah.” And they explained in Midrash Tehillim 52, “What do you benefit and what do you gain when you say evil speech? Is it not that you did not need money, for you had [already] become wealthy - as it is stated about him (I Samuel 21:8), ‘Saul’s chief herdsman.’ It was only because you preferred the evil to the good, and ‘the lie, to speaking truthfully’ - as you have removed His yoke.” And it is stated (Proverbs 6:30), “A thief is not held in contempt for stealing [to fill his hunger]; and it is written after it (Proverbs 6:32), “He who commits adultery is devoid of sense, etc.” This is meaning to say that he is worse than a thief, for [the former] needs to “fill his hunger.” Our Rabbis, may their memory be blessed, said (Midrash Tehillim 120) that one involved in evil speech is worse than both of them. For he does a great sin without benefit, as it is stated (Psalms 120:3), “What can you profit, what can you gain, O deceitful tongue?”

And secondly, for this [too] is one who speaks evil speech considered as if he denied a fundamental principle: For he says in his heart that his lips are in his [own] control; and because they do not do an action and he is in charge of his tongue, he determined in his heart that he should not restrain the spirit of his lips from saying that which comes to his spirit; and that only all the other limbs are not in his control to sin with them - like the matter that is stated (Psalms 12:5), “They say, “with lips such as ours, who can be our master?” And they do not say, “Where is the God that made me, to whom all the movements of His creations are given - not one is lacking. They are all subjected to doing His will. Rather, it is no matter - the petitions about the lips are in our control.” The evildoers that sin with other sins are not like this. For they know that their leaving God is bad and bitter, however they were pulled after their desire and their overpowering [evil] impulse; yet they are pained by this. And our Rabbis said (Arakhin 15b), “Evil speech [...] corresponds to three transgressions [and these are them]: Idol worship, forbidden sexual relations and bloodshed. [...] With regard to idol worship, it is stated (Exodus 32:31), ‘Oh, this people have sinned a great sin.’ With regard to forbidden sexual relations, it is stated (Genesis 39:9), ‘How can I do this great wickedness.’ With regard to bloodshed it is stated (Genesis 4:13), ‘My punishment is greater than I can bear.’ [And with regard to evil speech,] it is stated (Psalms 12:4), “May the Lord cut off all flattering lips, the tongue that speaks great things.”

And Chazal have said in Arachin 15b: "R. Yochanan said in the name of R. Yossi b. Zimra: If one speaks lashon hara, it is as if he would deny the Deity, as it is written (Psalms 12:5): 'Who have said: "With our tongue will we be mighty, our lips are with us. Who is L-rd over us?'" The reason is as they have written in the books of Yereim: It is known that one who wants to speak lashon hara looks all around to make sure that the one he wants to talk about is not there, as if the Eye above did not see, G-d forbid, as Chazal have said about a thief [(who steals at night)]. And in Midrash Tanchuma, Metzora 2: "[The sin of] lashon hara is grave; for one does not utter it until he denies the Deity."