Musar su Salmi 39:2

And consider how great is the sin of a man who speaks evil speech: Is their sin that they have sealed their lips and quieted their tongues from speaking words of Torah not big enough for them; that they surely [use them to] destroy with evil speech? And King David, peace be upon him, said (Psalms 119:23), “Though princes meet and speak against me, Your servant speaks out Your laws.” The explanation is that they are still from speaking out Your laws and speak evil speech and speak against me, while I am still speaking out Your laws. And our Rabbis said (Arakhin 15a) [that] the cure for evil speech - to save oneself from it - is to be involved with the Torah; as its stated (Proverbs 15:4), “A healing tongue is a tree of life,“ And this is [the meaning of] what is stated (Psalms 39:2), “I would keep my mouth muzzled, etc.” And our Rabbis, may their memory be blessed, explained (Midrash Tehillim 39) that the muzzle is involvement with Torah. And our Rabbis said (Yalkut Shimoni on Nach 721), “The Congregation of Israel is beloved by her voice and hated by her voice: She is beloved by her voice, as it is stated (Song of Songs 2:14), ‘let me hear your voice; for your voice is sweet.’ And she is hated by her voice, as it is stated (Jeremiah 12:8), ‘she raised her voice against Me; therefore I have hated her.’” And this is [the meaning of] what is stated (Proverbs 18:21), “Death and life are in the power of the tongue; those who love it will eat its fruit.” And its explanation is: “Those who love it will eat its fruit” - and one who loves the tongue, and that is a man that always wants to talk, for him the proper counsel is that he should, “eat its fruit.” That is, he should not speak idle words but rather words of Torah, wisdom and ethics and the bringing of peace between a man and his fellow; justify the [actions of] the masses; praise the good; disparage the evil; and be zealous about the truth. For there is no end to the merits that he can acquire for himself with his tongue. And [it is] as we discussed earlier, that life is in the tongue.

This episode teaches us that a person must not curse himself, since there are many "angels" who only wait for such opportunity of hearing someone cursing himself, i.e. his fate. All of this would not have happened to Moses had he not cursed himself. One part of his punishment was that his name is never mentioned in connection with the work of the Tabernacle, as is Aaron's or the names of the Princes at the time the Tabernacle was consecrated. The reason is that he had made the mistake of saying to G–d (even conditionally) "if not, wipe me out from Your book" (Exodus 32,32). We find that King David was extremely careful about this in Psalms 39,2, when he says: "I resolved I would watch my step, lest I offend by my speech. I will keep my mouth muzzled while the wicked man is in my presence." The "wicked man" that David refers to here is the angel who waits anxiously to respond "Amen," when man curses himself.

The man kept the dream in mind. He went to the king and he said to him, "My lord, the king, here is the milk of a bitch." The king got very angry and ordered that the man be hanged. As he was about to be hanged, all the limbs began to weep. Whereupon the tongue said to them, "Did I not tell you that you are not worth anything? If I save you, will you confess that I am king over all of you?" And they said to her, "Yes!" Then the man said to the executioners, "Take me back to the king, perhaps I can be saved." So they returned the man to the king, and the man asked, "Why did you order that I be hanged?" And the king said, "Because you brought me the milk of a bitch." The man said, "What difference does it make as long as you are healed? And moreover, a lioness is often called a bitch. They took the milk from him and tested it and it was found to be the milk of a lioness. Then all of the limbs said to the tongue, "Now we confess to you that you are king." And this is the meaning of the saying, "Death and life are in the power of the tongue." Therefore David said (Ps. 39:2), "I will take heed to my ways, that I sin not with my tongue" (Shohar Tov, 39:2—3, and Yalkut Tehillim, item 721).