Midrash for Pirkei Avot 4:11
רַבִּי אֱלִיעֶזֶר בֶּן יַעֲקֹב אוֹמֵר, הָעוֹשֶׂה מִצְוָה אַחַת, קוֹנֶה לוֹ פְרַקְלִיט אֶחָד. וְהָעוֹבֵר עֲבֵרָה אַחַת, קוֹנֶה לוֹ קַטֵּגוֹר אֶחָד. תְּשׁוּבָה וּמַעֲשִׂים טוֹבִים, כִּתְרִיס בִּפְנֵי הַפֻּרְעָנוּת. רַבִּי יוֹחָנָן הַסַּנְדְּלָר אוֹמֵר, כָּל כְּנֵסִיָּה שֶׁהִיא לְשֵׁם שָׁמַיִם, סוֹפָהּ לְהִתְקַיֵּם. וְשֶׁאֵינָהּ לְשֵׁם שָׁמַיִם, אֵין סוֹפָהּ לְהִתְקַיֵּם:
R. Eliezer ben Yaakov says: If one does one mitzvah, he acquires for himself one defense attorney [i.e., an angel speaking on his behalf.] And if one commits a sin, he acquires for himself one kategor (a prosecuting attorney), [i.e., an incriminating angel, (kategor being acronymic for "kore tigar," "calling out strife." Sanegor, the opposite of kategor is acronymic for "sone tigar," "hating strife.")] R. Yochanan Hasandlar says: Every gathering that is for the sake of Heaven is destined to endure, and (every gathering that is) not for the sake of Heaven is not destined to endure.
Pirkei DeRabbi Eliezer
REPENTANCE and good deeds are a shield against punishment. Rabbi Ishmael said: If repentance had not been created, the world would not stand. But since repentance has been created, the right hand of the Holy One, blessed be He, is stretched forth to receive the penitent every day, and He says, Repent, ye children of men. "Repent, ye children of men" (Ps. 90:8). Know thou the power of repentance. Come and see from Ahab, king of Israel, for he had robbed, coveted, and murdered, as it is said, "Hast thou killed, and also taken possession?" (1 Kings 21:19). He sent and called for Jehoshaphat, king of Judah, who gave him thrice daily forty stripes, and in fasting and with prayer he rose up early and retired late, before the Holy One, blessed be He, and he did not return any more to his evil deeds. His repentance was accepted, as it is said, || "Seest thou how Ahab humbleth himself before me? Because he humbleth himself before me, I will not bring the evil in his days" (1 Kings 21:29).