Guarding Speech Reveals Torah
בפסוק זכור את א"ע ה"א למרים כו' דרשו חז"ל שהוא אזהרה ללשון הרע
Regarding the verse "Remember what Hashem, your God, did to Miriam" (Devarim 24:9), Chazal expounded that this is a warning against lashon hara, evil speech.
The Torah's command to remember what happened to Miriam is understood by Chazal as a standing warning against speaking lashon hara.
והענין הוא כי מאחר שכחן של בנ"י בפה והוא בחי' תורה שבע"פ והוא הבאר שניתנה בזכות מרים
The matter is as follows: since the strength of Bnei Yisrael lies in the mouth, and this is the aspect of Torah she'be'al peh, the Oral Torah, and this is the well that was given in the merit of Miriam,
The unique power of Bnei Yisrael is expressed through speech, which corresponds to the Oral Torah, and this in turn is linked to Miriam's well that sustained them in the wilderness.
וכ' והאבן גדולה ע"פ הבאר
and it is written, "and the stone upon the mouth of the well was great" (Bereishis 29:2),
The Sfas Emes brings the verse describing the great stone covering the mouth of the well in Yaakov's encounter.
יתכן לרמוז על חטא לה"ר שדרשו חז"ל שנק' גדולה כמ"ש לשון מדברת גדולות
it is fitting to allude to the sin of lashon hara, which Chazal expounded is called "great," as it is written, "a tongue that speaks great things" (Tehillim 12:4).
That "great" stone hints at the sin of lashon hara, which is itself described as "great" because the tongue speaks of great things.
ולכן ע"י שמירת הפה יכולין לבא לבחי' הבאר וע"י שמרים הי' בחי' הנ"ל הצריכה שמירה ביותר וכ' מוצא שפתיך תשמור שכפי רוב קדושת האבר והמקום כן צריך שמירה ביותר
Therefore, through guarding the mouth one is able to come to the aspect of the well; and since Miriam was the aforementioned aspect, she required guarding all the more, and it is written, "that which goes forth from your lips you shall guard" (Devarim 23:24) — for according to the great kedushah of the limb and the place, so too it requires that much more guarding.
Because the mouth is the seat of such holy power, it demands extra guarding; the holier the limb and the place, the greater the vigilance required, as the verse instructs us to guard what leaves our lips.
וע"י התחברות בנ"י שהיא היפך לה"ר ושנאת חנם זוכין להתגלות תורה שבע"פ כמ"ש ונאספו שמה כו' וגללו את האבן
And through the joining together of Bnei Yisrael, which is the opposite of lashon hara and sinas chinam, baseless hatred, they merit the revelation of Torah she'be'al peh, as it is written, "and they would all gather there... and they would roll the stone" (Bereishis 29:3).
When Bnei Yisrael unite in harmony, the antidote to lashon hara and baseless hatred, they remove the obstructing stone and reveal the Oral Torah, just as the shepherds gathered to roll away the well's stone.
[אבנים הם אותיות וכמו שיש בקדושה כן יש זה לעומת זה דברי הבל ובנינין דשקר]:
[The stones are letters; and just as there is this in the realm of kedushah, so too, opposite it, on the other side, there are words of emptiness and structures of falsehood.]
Letters are likened to stones, and just as holy speech builds structures of kedushah, the opposing side produces empty words and false structures.
Summary: The Sfas Emes teaches that the verse commanding us to remember what happened to Miriam is a warning against lashon hara, rooted in the truth that the strength of Bnei Yisrael lies in the mouth, the source of Torah she'be'al peh and of Miriam's well. The "great stone" upon the mouth of the well hints at the sin of evil speech, which is itself called "great," for the tongue speaks great things; thus the very organ of holiness demands the greatest guarding. By guarding the mouth, one draws close to the wellsprings of the Oral Torah. When Bnei Yisrael join together in unity, the opposite of lashon hara and sinas chinam, they merit to roll away the stone and reveal the Oral Torah. For letters are like stones: just as holy speech builds edifices of kedushah, the opposing side builds only words of emptiness and structures of falsehood.