Guarding Speech Binding Soul
בפסוק השמר בנגע הצרעת כו' זכור א"א עשה כו' למרים כו' בצאתכם ממצרים
On the pasuk "Take heed in the affliction of tzaraas etc. Remember what Hashem your God did to Miriam on the way, when you came out of Mitzrayim" (Devarim 24:8-9).
The Sfas Emes opens with the pasuk that places guarding against tzaraas alongside the command to remember what happened to Miriam, who was stricken with tzaraas for speaking against Moshe Rabbeinu.
כ' בזה שמירה וזכירה לומר שהתורה כולה תלוי' בפה כמ"ש מות וחיים ביד לשון ובגמ' איתא יכרת ה' שפתי חלקות לשון מד' גדולות כו' המספר לה"ר מרבה חטאים כמו ע"ז וג"ע וש"ד ע"ש בערכין הענין הוא כי אלה הג' הם פוגמים בנפש רוח נשמה לכן צריכין למס"נ עליהם
Scripture wrote here both "shemirah" (guarding) and "zechirah" (remembering), to teach that the entire Torah is dependent upon the mouth, as it is written "Death and life are in the hand of the tongue" (Mishlei 18:21). And in the Gemara it is brought, "May Hashem cut off all smooth lips, the tongue that speaks great things" (Tehillim 12:4) — one who speaks lashon hara multiplies sins like avodah zarah, gilui arayos, and shefichus damim (idolatry, forbidden relations, and bloodshed); see there in Arachin. The matter is that these three damage the nefesh, ruach, and neshamah, and therefore one is required to give up his life rather than transgress them.
The Torah uses both "guarding" and "remembering" to show that all of Torah hinges on the mouth; lashon hara is weighed against the three cardinal sins because it, too, damages the three levels of the soul — nefesh, ruach, and neshamah.
ובחי' הפה הוא תיקון הגוף לקבל הארת נר"נ לכן איתא ל"מ לגוף טוב משתיקה שע"י שמירת הפה שזה יתרון האדם ויפח באפיו כו' לנפש חי' רוח ממללא ע"י מאיר ומתקשר הנפש בגוף לכן שומר פיו ולשונו ש' מצרות נפשו כי הנפש בגוף כמו במאסר ובנ"י ביצ"מ יצאו מזה המאסר לכן צריכין לשמור הפה ומספר לה"ר כ' יכרת ה' שנכרת הנפש מהגוף
And the aspect of the mouth is the rectification of the body, enabling it to receive the illumination of the nefesh, ruach, and neshamah; therefore it is brought that nothing is better for the body than silence, for through guarding the mouth — which is the distinction of man, as it says "and He breathed into his nostrils a living soul" (Bereishis 2:7), which Onkelos renders "a speaking spirit" — through this the nefesh shines and binds itself to the body. Therefore one who guards his mouth and his tongue guards his soul from troubles (Mishlei 21:23), for the nefesh within the body is like one in prison, and Bnei Yisrael at yetzias Mitzrayim went out from this prison; therefore they must guard the mouth. And concerning one who speaks lashon hara it is written "May Hashem cut off" — that the nefesh is cut off from the body.
The mouth is the tool that fixes the body so it can receive the light of the soul; since speech is what makes man a "living, speaking being," guarding one's speech keeps the neshamah bound to the body, while lashon hara severs the soul from the body just as the cardinal sins do.
כמ"ש מו"ח ביד לשון
As it is written, "Death and life are in the hand of the tongue" (Mishlei 18:21).
He returns to the proof-text that life and death rest in the power of the tongue, sealing the point that speech holds the soul's fate.
ודו"ק כי קצרתי:
And examine this closely, for I have been brief.
He closes by inviting the reader to ponder the idea further, since he expressed it only briefly.
Summary: The Sfas Emes explains why the Torah pairs guarding against tzaraas with remembering Miriam, both phrased in terms of "guarding" and "remembering": the whole Torah depends on the mouth, for "death and life are in the hand of the tongue." Lashon hara is equated with the three cardinal sins of avodah zarah, gilui arayos, and shefichus damim because all four damage the three levels of the soul — nefesh, ruach, and neshamah — for which one must give up his life. The mouth is the body's rectification, allowing it to receive the soul's light, since speech is the very thing that makes man a "living, speaking being" and binds the neshamah to the body. Thus one who guards his speech guards his soul, releasing it from the prison of the body just as Bnei Yisrael were released at yetzias Mitzrayim, whereas one who speaks lashon hara causes the soul to be "cut off" from the body.