Guarding Speech Empowers Vows
בפסוק מוצא שפתיך תשמור ודרשו חז"ל מ"ש מ"ע תשמור מל"ת כו'
On the pasuk "That which has gone forth from your lips you shall observe" (Devarim 23:24), Chazal expounded: from where do we derive that a positive commandment must be guarded? From the wording "you shall observe," which is the language used for a prohibition.
The Torah uses the language of a prohibition ("you shall observe") even for a positive commandment, teaching that fulfilling what your lips have pledged is itself a binding obligation.
כי הנדר והקבלה של מצוה היא מסייע הרבה אל האדם
For the vow and the taking-on of a mitzvah upon oneself is a great help to a person.
When a person verbally takes a mitzvah upon himself through a vow or commitment, that act of acceptance gives him real strength to carry it through.
לכן איתא שנשבעין לקיים המצוה שנא' נשבעתי ואקיימה
Therefore it is taught that one may take an oath to fulfill a mitzvah, as it says, "I have sworn, and I will fulfill it" (Tehillim 119:106).
Because the spoken commitment is such a help, Chazal sanctioned swearing an oath to perform a mitzvah, as Dovid HaMelech did.
וכן מקבלין בנ"י ב"פ בכל יום עול מלכותו ית'
And similarly Bnei Yisrael accept upon themselves twice each day the yoke of His Kingship, may He be blessed.
The twice-daily acceptance of the yoke of Heaven in Krias Shema is exactly this kind of strengthening verbal commitment.
וקבלה זו מסייעת אל האדם אך זה תליא כפי שמירת הפה מדברי הבלי עוה"ז
And this acceptance is a help to a person, but it depends on how well he guards his mouth from the empty words of this world.
The power of such an acceptance to truly help a person hinges on how carefully he guards his mouth from the empty chatter of this world.
כן יש כח בפה לקבל ושיעשה דיבורו רושם לכן צריכין לשמור הפה ומקפידין מאוד על לשון הרע
Just so, there is a power in the mouth to accept and to make one's speech leave a lasting impression; therefore one must guard the mouth, and one must be exceedingly careful regarding lashon hara.
Because the mouth has the power to make one's words take real effect, one must guard it carefully and be especially vigilant about lashon hara.
וכמ"ש זכור אשר עשה ה"א למרים כו' וכ' בצאתכם ממצרים ע"י שאחר יצ"מ ניתן כח הפה והקול לבנ"י כמ"ש במ"א פי' אנכי ה"א המעלך מא"מ הרחב פיך כו' וכן הי' מתן תורה אחר יצ"מ
As it is written, "Remember what Hashem your God did to Miriam" (Devarim 24:9), and it is written there "when you came out of Mitzrayim" (Devarim 24:9) — because after Yetzias Mitzrayim the power of the mouth and the voice was given to Bnei Yisrael, as is explained elsewhere on the meaning of "I am Hashem your God who brought you up from the land of Mitzrayim; open wide your mouth" (Tehillim 81:11), and likewise Mattan Torah was after Yetzias Mitzrayim.
The Torah's warning about Miriam's lashon hara is tied to Yetzias Mitzrayim, because it was after the Exodus that Bnei Yisrael received the power of speech and voice, the same power that made Mattan Torah possible.
ולכן מאז צריכין מאוד לשמור הפה
And therefore, from that time onward, one must guard the mouth exceedingly.
Since that elevated power of speech was given at the Exodus, from that point on the obligation to guard the mouth became especially weighty.
וכפי השמירה מל"ת כן מסייע הפה לקיים המ"ע
And according to the measure of one's guarding against prohibitions, so too does the mouth help one to fulfill the positive commandments.
The more diligently a person keeps his mouth from forbidden speech, the more his mouth becomes an instrument that helps him fulfill the positive commandments.
ולכן צריכין לקיים הנדר
And therefore one must fulfill the vow.
Because of all this, a person is obligated to actually carry out the vow he has made.
ואז מקבלין שכר על הקבלה בפ"ע ועל הקיום בפ"ע
And then one receives reward for the taking-on by itself, and for the fulfillment by itself.
When he does fulfill it, he earns two distinct rewards: one for the act of taking it on, and a separate one for the act of fulfilling it.
ולעומת זה כ' ג"כ דרוש ידרשנו דרישה על הנדר כיון שלא קיים הי' הקבלה עבירה ודרישה על שלא קיים הנדר
And correspondingly it is also written, "He will surely require it" (Devarim 23:22) — there is a demand made concerning the vow, since, having not fulfilled it, the taking-on became a transgression, and there is a demand concerning his not having fulfilled the vow.
Conversely, when a person fails to fulfill his vow, the unfulfilled commitment itself becomes a transgression, and he is held to account both for the failed acceptance and for the failure to fulfill.
והאמת כי אם הי' הקבלה כראוי וודאי הי' מקיים והוי זה סימן שלא קיבל עליו בלב שלם
And the truth is that had the taking-on been as it ought to be, he would certainly have fulfilled it; and this is a sign that he did not accept it upon himself with a whole heart.
Had the acceptance been wholehearted and proper, he surely would have carried it out; failing to do so reveals that his heart was never fully in it.
וכ' בקהלת כאשר תדור כו' לאלקים כו' לשלמו כי אין חפץ בכסילים כו' פי' כי אם שהרצון והקבלה שנק' חפץ הוא חשוב מאד בשמים אבל לא בכסילים שאין מקיימים
And it is written in Koheles, "When you vow a vow to God, do not delay to pay it, for He has no desire in fools" (Koheles 5:3-4) — the explanation being that, while the desire and the taking-on, which is called "chefetz" (desire), is exceedingly precious in Heaven, this is not so with fools, who do not fulfill what they accept.
Koheles teaches that the will and commitment behind a vow, called "chefetz," is exceedingly precious in Heaven — but only for those who follow through, not for fools who fail to fulfill what they pledged.
וגם אינו כלל רצון אמת וזה אין חפץ בכסילים
And moreover, theirs is not at all a true will, and this is the meaning of "He has no desire in fools."
The fool's pledge is not even a genuine will to begin with, which is why Hashem has no "desire" in it.
מכלל שהמקיים גם הקבלה חביב לפניו ית':
From this we infer that with one who fulfills, even the taking-on is beloved before Him, may He be blessed.
By contrast, for one who does fulfill his vow, even the initial act of accepting it is beloved before Hashem.
Summary: The Sfas Emes explains why the Torah uses the language of a prohibition to describe the obligation to keep one's verbal pledges: the act of taking a mitzvah upon oneself through a vow or commitment is itself a powerful aid to a person, just as Bnei Yisrael are strengthened by accepting the yoke of Heaven twice daily. The potency of this verbal acceptance, however, depends entirely on how carefully one guards the mouth from empty and forbidden speech, especially lashon hara — for it was at Yetzias Mitzrayim that the elevated power of speech and voice was given to Bnei Yisrael, the very power that made Mattan Torah possible. When a person fulfills his vow he earns a double reward, one for the acceptance and one for the fulfillment; but when he fails, the unfulfilled commitment turns into a transgression, revealing that his heart was never wholly in it. Citing Koheles, the Sfas Emes teaches that the will and commitment behind a vow is exceedingly precious in Heaven only for one who follows through, since the fool's pledge is not even a genuine will. Thus, for the one who truly fulfills, even the initial taking-on of the mitzvah is beloved before Hashem.