The starting point is Mishlei's own repeated insistence that speech is a lethal and life-giving force: Mishlei 18:21 states that death and life are in the hand of the tongue, and this is echoed by Mishlei 13:3 and Mishlei 21:23 on guarding the mouth to preserve life, by Mishlei 10:19 on the sin inherent in excessive words, by Mishlei 12:18 contrasting sword-like speech with healing speech, by Mishlei 15:1 on how a gentle answer turns away wrath, by Mishlei 17:9 on how repeating a matter alienates friends, and by Mishlei 25:11 praising the well-turned word.
The Gemara intensifies this theme into an almost physical account of the tongue's danger: Arakhin 15b describes God surrounding the tongue with two walls of bone and flesh precisely because of its destructive capacity, a reading elaborated by Netivot Olam, Netiv Halashon 1:2 on how the tongue kills three people and Sefer Chasidim 34, which cites the same teaching alongside the imperative to multiply silence. Netivot Olam, Netiv Halashon 1:3 further explains that speech, unlike the hand, is uniquely suited to bring life and death to actual fulfillment, Sulam on Zohar, Pekudei 875 describes an impure spirit aroused by lashon hara that brings death and sword into the world, and Arakhin 16b attributes the leper's isolation to his having separated husband from wife and person from person through malicious speech, while Mekhilta DeRabbi Yishmael, Tractate Kaspa 2:10 extends the prohibition to one who merely accepts a false report.
A broad ethical-mystical literature builds on this to describe the cosmic and personal stakes of every word: Nefesh HaChayim, Gate I 13:2 teaches that no utterance is wasted but builds or destroys worlds above, Tzidkat HaTzadik 234 frames constant Torah speech as the remedy for man's compulsive need to speak, Ner Mitzvah, Volume I:27 grounds this in man's defining nature as a speaking being, and Even Sheleimah 7:1 states that guarding the mouth protects the soul more than any fast or affliction. Orchot Tzadikim 21:4 categorizes speech into types of harm and benefit, Sefer Chasidim 39 distinguishes when one must speak from when one must stay silent, Sefat Emet, Devarim, Sukkot 22:9 and Sefat Emet, Devarim, Ki Teitzei 3 link the mouth's inner sanctity to the commandment of Torah study, Tanya 30 and Tanya 32 press the struggle against habitual gossip even about true and minor faults, Recanati on the Torah, Kedoshim:8 describes letters leaving the mouth as making an actual mark in the air, and Drashot Maharal, Drush L'Shabbat T'Shuva:37 insists that lashon hara, though seemingly light, carries grave punishment.
The codifiers translate this severity into binding law: Mishneh Torah, Human Dispositions 7:1-8 rules that gossip-gathering is a Torah violation that can cause deaths even when the statements are true, Chafetz Chaim 1:1-4 rules that speaking demeaningly of another, even if entirely true, is forbidden lashon hara, and Sefer HaMitzvot, Negative Commandments 304 and Sefer HaMitzvot, Negative Commandments 301 together with Sefer HaChinukh 236:1-3 codify the prohibitions against vengeance and talebearing as distinct negative commandments. Shulchan Arukh, Choshen Mishpat 228:1 rules that verbal wronging (onaat devarim) is in fact worse than monetary wronging, since words cannot be repaid.
Several narrative sugyot show the practical consequences of speech gone wrong: Midrash Tanchuma, Sh'lach 9 reads the spies' report as the paradigm of lashon hara that opens with praise and ends in ruin, Megillah 28a warns never to take an ordinary person's curse lightly since even Avimelech's curse of Sarah was fulfilled, Bava Metzia 59a rules it is better to sin privately than to humiliate another in public, Sotah 42a records that the war-anointed priest addressed the people in the holy tongue, and Yoma 86a shows how a public figure's casual word or deed in commerce can constitute chillul Hashem.
A cluster of lesser-known practical rulings about speech emerges across the sources: Berakhot 6b rules that one who benefits from a wedding feast without gladdening the groom with words violates five voices of simcha; Shabbat 119b teaches that reciting vayechulu makes a person a partner with God in creation through speech; Mishneh Torah, Repentance 2:9 rules that even mere hurtful words between people require active appeasement before forgiveness is granted; Shulchan Arukh, Orach Chayim 156 and Shulchan Arukh HaRav, Orach Chayim 156:1-2 require that one's business speech never deviate from the truth; Maamar Mordechai Hilchot Taaniyot U'Vein HaMetzarim 15:10 cites the ruling that one's manner of speech on Shabbat may not be the same as on a weekday; Mishnah Berurah 185:1-3 rules that Birkat Hamazon must be said with the lips and not merely thought; Avnei Yashfeh 7:124 holds that one may not repeat something told to him by another without permission, derived from the wording of Yoma 4b; Ran on Pesachim 4a:5 records the tradition not to speak between eating matzah and the start of the meal so as not to lose concentration; Megillah 28b prescribes reciting a verse or halacha as the proper pretext for summoning someone out of a synagogue; Kiddushin 33b discusses standing before a Torah scroll and before those who study it; and Shabbat 104b and Taanit 7a touch respectively on the halachic definition of writing letters on Shabbat and on the Torah's words being likened to water and dew.
Modern writers carry the theme forward: Gray Matter IV, Family and Community Matters reaffirms that lashon hara and rechilut remain forbidden even between close friends and spouses, Peninei Halakhah, Likkutim II 2:9 and Peninei Halakhah, Likkutim II 2:12 describe speech as the mark of human uniqueness that must be treated with reverence and truthfulness, Steinsaltz on Yeshayahu 57:19 reads God as the Creator of speech who grants healing words to the suffering, Dirshuni II, Adultery, II 16 reads Esther's turning point as the moment she takes hold of the power of speech, Birkat Asher on Torah, Bereishit 2:7:1 questions why man was given the power of speech before having anyone to speak to, and Talks of Yeshayahu Leibowitz on the Parasha, Introduction 3:39 invokes man's advantage over the beast — the power of speech — as preferable to weapons.
Among responsa directly addressing contemporary practice, Mishneh Halachos 13:23 prohibits idle conversation in a synagogue or beit midrash, particularly during prayer or Torah study, holding that it diminishes the sanctity of the place, interrupts the learning taking place there, and risks drawing others into the conversation and thereby into sin.