If I’m not mistaken, I read about such a mitzvah in Mesechet Taanit, but then I couldn’t find it and now it’s annoying me. Though, I could have also simply seen it somewhere else. So does anyone know the answer?
This is perhaps a bit more general than you want, but Rabbenu Yonah, in his definitive treatise on repentance (שערי תשובה – Shaarei Teshuvah) details twenty steps towards a full repentance, the tenth of which is:
Act in a manner contrary to the sin.
Indeed, the Midrash says:
If you have done piles of sins, do piles and piles of commandments that correspond to them. [Leviticus Rabbah 21:5]
"What’s the mitzvah or deed you can do that could absolve you of a sin you did 70 years ago?"
the mitzvah of Teshuva
[which is actually debatable if its a mitzvah, or higher than mitzvos]
has the power to rectify any sin,no matter how long ago it was performed, as the Rambam [teshuva 2:1] clearly states:
"If he does not repent until his old age, at a time when he is incapable of doing what he did before, even though this is not a high level of repentance, he is a Baal-Teshuvah.
Even if he transgressed throughout his entire life and repented on the day of his death and died in repentance, all his sins are forgiven as [Ecclesiastes, op. cit.:2] continues: "Before the sun, the light, the moon, or the stars are darkened and the clouds return after the rain..." - This refers to the day of death. Thus, we can infer that if one remembers his Creator and repents before he dies, he is forgiven."
So, what is Teshuva?
The Alter Rebbe in Igros Hateshuva writes that the basic requirement is only to abandon the sin [and the fact that rambam [there halacha 2] mentions confession is a side point, as mentioned at the end here and in note 34]:
Now, the Mitzvah of repentance34 as required by the Torah is simply the abandonment of sin
וְהִנֵּה, מִצְוַת הַתְּשׁוּבָה מִן הַתּוֹרָה, הִיא עֲזִיבַת הַחֵטְא בִּלְבַד
(35cf. Sanhedrin, ch. 336; Choshen Mishpat, end of Sec. 34,37 regarding testimony38), where it is stated that if a potential witness simply abandons and does not repeat the transgression that had previously disqualified him, he is once again able to testify.39
(כִּדְאִיתָא בַּגְּמָרָא פֶּרֶק ג' דְּסַנְהֶדְרִין וּבְחֹשֶׁן־מִשְׁפָּט סוֹף סִימָן ל"ד לְעִנְיַן עֵדוּת),
This means that he must resolve in perfect sincerity never again to revert to folly, to rebel against G‑d’s rule;
דְּהַיְינוּ, שֶׁיִּגְמוֹר בְּלִבּוֹ בְּלֵב שָׁלֵם, לְבַל יָשׁוּב עוֹד לְכִסְלָה, לִמְרוֹד בְּמַלְכוּתוֹ יִתְבָּרֵךְ,
he will never again violate the King’s command, G‑d forbid, neither a positive command40 nor a prohibition.41
וְלֹא יַעֲבוֹר עוֹד מִצְוֹת הַמֶּלֶךְ חַס וְשָׁלוֹם, הֵן בְּמִצְוֹת עֲשֵׂה הֵן בְּמִצְוֹת לֹא תַעֲשֶׂה.
This is the basic meaning of the term teshuvah (“repentance”)—to return to G‑d with all one’s heart and soul, to serve Him, and to observe all His commandments,
וְזֶהוּ עִיקַּר פֵּירוּשׁ לְשׁוֹן "תְּשׁוּבָה" לָשׁוּב אֶל ה' בְּכָל לִבּוֹ וּבְכָל נַפְשׁוֹ, לְעָבְדוֹ וְלִשְׁמוֹר כָּל מִצְוֹתָיו,
for so does Scripture state: “Let the wicked abandon his path, and the sinful his thoughts, and return to G‑d….”42
כְּמוֹ שֶׁכָּתוּב: "יַעֲזוֹב רָשָׁע דַּרְכּוֹ וְאִישׁ אָוֶן מַחְשְׁבוֹתָיו, וְיָשׁוֹב אֶל ה' וְגוֹ'";
In the Torah portion of Nitzavim43 it is likewise written:44 “You shall return unto the Lord your G‑d and hearken to His voice…with all your heart….”45
וּבְפָרָשַׁת נִצָּבִים כְּתִיב: "וְשַׁבְתָּ עַד ה' אֱלֹקֶיךָ וְשָׁמַעְתָּ בְקוֹלוֹ וְגוֹ', בְּכָל לְבָבְךָ וְגוֹ'",
[So, too:] “Return, O Israel, unto the L-rd your G‑d…”46; [and elsewhere:] “Bring us back, O L-rd, unto You….”47
"שׁוּבָה יִשְׂרָאֵל עַד ה' אֱלֹקֶיךָ וְגוֹ'", "הֲשִׁיבֵנוּ ה' אֵלֶיךָ וְגוֹ'".
Repentance, then, entails returning to G‑d, performing His commandments, and refraining from sin.
This differs from the popular conception that repentance is synonymous with fasting on account of one’s sins.
וְלֹא כְּדַעַת הֶהָמוֹן, שֶׁהַתְּשׁוּבָה הִיא הַתַּעֲנִית.
Even in the case of sins punishable by excision or execution, where atonement is made complete by suffering, as previously quoted from the Baraita in Yoma,
וַאֲפִילוּ מִי שֶׁעָבַר עַל כְּרֵיתוֹת וּמִיתוֹת בֵּית־דִּין, שֶׁגְּמַר כַּפָּרָתוֹ הִיא עַל־יְדֵי יִסּוּרִים,
this means that it is G‑d Who brings suffering upon the sinner in order to complete his atonement
הַיְינוּ, שֶׁהַקָּדוֹשׁ־בָּרוּךְ־הוּא מֵבִיא עָלָיו יִסּוּרִים
(48as the verse clearly specifies, “With a rod shall I remember [their sin]”).
(וּכְמוֹ שֶׁכָּתוּב: "וּפָקַדְתִּי בְשֵׁבֶט וְגוֹ'" – "וּפָקַדְתִּי" דַיְיקָא),
That is to say: When G‑d finds his repentance acceptable, as he returns to Him with all his heart and soul, out of love,
וְהַיְינוּ, כְּשֶׁתְּשׁוּבָתוֹ רְצוּיָה לְפָנָיו יִתְבָּרֵךְ, בְּשׁוּבוֹ אֶל ה' בְּכָל לִבּוֹ וְנַפְשׁוֹ מֵאַהֲבָה,
then following the initiative undertaken from below, and “as water reflects the countenance…,”49 there is an awakening Above, arousing G‑d’s love and kindness, to scour his sin and entirely cleanse him of it through affliction in This World,
אֲזַי, בְּאִתְעָרוּתָא דִלְתַתָּא וְ"כַּמַּיִם הַפָּנִים וְכוּ'" – אִתְעָרוּתָא דִלְעֵילָּא, לְעוֹרֵר הָאַהֲבָה וְחֶסֶד ה' לְמָרֵק עֲוֹנוֹ בְּיִסּוּרִים בָּעוֹלָם הַזֶּה,
in the spirit of the verse, “For he whom the L-rd loves, He chastises….”50
וּכְמוֹ שֶׁכָּתוּב: "כִּי אֶת אֲשֶׁר יֶאֱהַב ה' יוֹכִיחַ וְגוֹ'".
This is something quite different from any fasts or afflictions that an individual undertakes himself.
It is for this reason that the Rambam and Sefer Mitzvot Gadol51 make no mention whatsoever of fasting as related to the mitzvah of repentance, even in the case of sins punishable by excision or capital sins.
וְלָכֵן לֹא הִזְכִּירוּ הָרַמְבַּ"ם וְהַסְּמַ"ג שׁוּם תַּעֲנִית כְּלָל בְּמִצְוַת הַתְּשׁוּבָה, אַף בִּכְרֵיתוֹת וּמִיתוֹת בֵּית־דִּין,
I.e., fasting is not required even with regard to those sins whose atonement is completed through suffering.
They cite only confessing [verbally] and requesting forgiveness, as the Torah prescribes, “They shall confess their sin….”52
רַק הַוִּידּוּי וּבַקָּשַׁת מְחִילָה, כְּמוֹ שֶׁכָּתוּב בַּתּוֹרָה: "וְהִתְוַדּוּ אֶת חַטָּאתָם וְגוֹ'".
Why are confession and requesting forgiveness indeed part of repentance?
Every sin consists of a body and a soul. The actual misdeed itself is the “body” of the sin, and the bodily pleasure and ensuing desire with which it was committed are its “soul.” Repentance involves eliminating both these elements.
The “soul” of the sin is eradicated by the earnest regret of the individual, who is mortified and pained by his past. Inasmuch as pain is the opposite of pleasure, it negates the pleasure which had earlier aroused his desire to sin and thereby obliterates the “soul” of the sin.
However, the “body” of the sin also needs to be nullified. Simply refraining from further transgression lacks the action that would negate the sinful act itself, its “body.” This is accomplished through verbal confession, for “verbalization is also considered to be an action.”53
At any rate, verbal confession is thus a component of repentance while fasting is not.