Berakhot 28b
ברכות כ״ח ב — ד"ה תָּנוּ רַבָּנַן
Berakhot 28b:23
The Mishnah records Rabban Shimon ben Gamliel’s account that Shmuel ha-Katan instituted the blessing concerning informers, which was added to the Amidah and brought the standard weekday Amidah to nineteen blessings. This is the central rabbinic source for who composed the nineteenth blessing.
תָּנוּ רַבָּנַן: שִׁמְעוֹן הַפָּקוֹלִי הִסְדִּיר שְׁמוֹנֶה עֶשְׂרֵה בְּרָכוֹת לִפְנֵי רַבָּן גַּמְלִיאֵל עַל הַסֵּדֶר בְּיַבְנֶה. אָמַר לָהֶם רַבָּן גַּמְלִיאֵל לַחֲכָמִים: כְּלוּם יֵשׁ אָדָם שֶׁיּוֹדֵעַ לְתַקֵּן בִּרְכַּת הַמִּינִים? עָמַד שְׁמוּאֵל הַקָּטָן וְתִקְּנָהּ. לְשָׁנָה אַחֶרֶת שְׁכָחָהּ,
In light of the previous mention of the blessing of the heretics, the Gemara explains how this blessing was instituted: The Sages taught: Shimon HaPakuli arranged the eighteen blessings, already extant during the period of the Great Assembly, before Rabban Gamliel, the Nasi of the Sanhedrin, in order in Yavne. Due to prevailing circumstances, there was a need to institute a new blessing directed against the heretics. Rabban Gamliel said to the Sages: Is there any person who knows to institute the blessing of the heretics, a blessing directed against the Sadducees? Shmuel HaKatan, who was one of the most pious men of that generation, stood and instituted it. The Gemara relates: The next year, when Shmuel HaKatan served as the prayer leader, he forgot that blessing,