Yoma8: 9
הָאוֹמֵר: ,,אֶחֱטָא וְאָשׁוּב; אֶחֱטָא וְאָשׁוּב”, אֵין מַסְפִּיקִין בְּיָדוֹ לַעֲשׂוֹת תְּשׁוּבָה. ,,אֶחֱטָא וְיוֹם הַכִּפּוּרִים מְכַפֵּר”, אֵין יוֹם הַכִּפּוּרִים מְכַפֵּר. עֲבֵרוֹת שֶׁבֵּין אָדָם לַמָּקוֹם, יוֹם הַכִּפּוּרִים מְכַפֵּר; עֲבֵרוֹת שֶׁבֵּין אָדָם לַחֲבֵרוֹ, אֵין יוֹם הַכִּפּוּרִים מְכַפֵּר, עַד שֶׁיְּרַצֶּה אֶת חֲבֵרוֹ. אֶת זוֹ דָּרַשׁ רַבִּי אֶלְעָזָר בֶּן עֲזַרְיָה: ,,מִכֹּל חַטֹּאתֵיכֶם לִפְנֵי ה' תִּטְהָרוּ” — עֲבֵרוֹת שֶׁבֵּין אָדָם לַמָּקוֹם, יוֹם הַכִּפּוּרִים מְכַפֵּר; עֲבֵרוֹת שֶׁבֵּין אָדָם לַחֲבֵרוֹ, אֵין יוֹם הַכִּפּוּרִים מְכַפֵּר, עַד שֶׁיְּרַצֶּה אֶת חֲבֵרוֹ. אָמַר רַבִּי עֲקִיבָא: אַשְׁרֵיכֶם יִשְׂרָאֵל! לִפְנֵי מִי אַתֶּם מִטַּהֲרִין? וּמִי מְטַהֵר אֶתְכֶם? אֲבִיכֶם שֶׁבַּשָּׁמַיִם! שֶׁנֶּאֱמַר: ,,וְזָרַקְתִּי עֲלֵיכֶם מַיִם טְהוֹרִים וּטְהַרְתֶּם”; וְאוֹמֵר: ,,מִקְוֵה יִשְׂרָאֵל ה' ”. מָה מִקְוֶה מְטַהֵר אֶת הַטְּמֵאִים, אַף הַקָּדוֹשׁ בָּרוּךְ הוּא מְטַהֵר אֶת יִשְׂרָאֵל.
He who says, “I will sin and repent; I will sin and repent,” they do not assist him in achieving repentance.  [If he says] “I will sin and Yom Kippur will provide atonement,” Yom Kippur does not provide atonement.  For sins between man and God, Yom Kippur provides atonement; but for sins between man and his fellow man, Yom Kippur does not provide atonement, until he appeases his fellow man.  This did R' Elazar ben Azariah expound: From all of your sins before God shall you be cleansed —- [from this we derive] for sins between man and God, Yom Kippur provides atonement; but for sins between man and his fellow man, Yom Kippur does not provide atonement, until he appeases his fellow man.  R' Akiva said: Praiseworthy are you, O Israel! Before Whom do you cleanse yourselves? Who cleanses you? Your Father in Heaven! As it is said: And I will sprinkle pure water upon you and you shall be cleansed; and He also says: The mikveh of Israel is Hashem. Just as a mikveh purifies the contaminated, so does the Holy One, Blessed is He, purify Israel.