אורח חיים קי״גOrach Chayim 113

Se’if 1א׳

אֵלּוּ בְּרָכוֹת שֶׁשּׁוֹחִין בָּהֶם: בְּאָבוֹת תְּחִלָּה וָסוֹף, וּבְהוֹדָאָה תְּחִלָּה וָסוֹף; וְאִם בָּא לִשְׁחוֹת בְּסוֹף כָּל בְּרָכָה, אוֹ בִּתְחִלָּתָהּ, מְלַמְּדִין אוֹתוֹ שֶׁלֹּא יִשְׁחֶה; אֲבָל בְּאֶמְצָעִיתָן, יָכוֹל לִשְׁחוֹת.

The Laws of Bowing in the Eighteen Blessings [i.e. Amidah]. Containing 9 S'ifim These are the blessings in which we bow: in Avot [the first blessing], [at the] beginning and end; in Hoda-a [the second-to-last blessing], [at the] beginning and end. And if one comes to bow at the end of every blessing or at its beginning, we teach [that person] that one does not bow, but in their [i.e. the blessings'] middles, one may bow.

Se’if 2ב׳

הַנּוֹהֲגִים לִשְׁחוֹת בְּרֹאשׁ הַשָּׁנָה וְיוֹם כִּפּוּר כְּשֶׁאוֹמְרִים זָכְרֵנוּ וּמִי כָמוֹךָ, צְרִיכִים לִזְקֹף כְּשֶׁמַּגִּיעִים לְסוֹף הַבְּרָכָה. הַגָּה: וְאַף עַל גַּב דִּבְאָבוֹת כּוֹרֵעַ בְּסוֹף הַבְּרָכָה, מִכָּל מָקוֹם צָרִיךְ לִזְקֹף מְעַט בְּסוֹף זָכְרֵנוּ, כְּדֵי שֶׁיְּהֵא נִרְאֶה שֶׁחוֹזֵר וְכוֹרֵעַ מִשּׁוּם חִיּוּב (דִּבְרֵי עַצְמוֹ לפי' הַטּוּר).

Those who have the custom to bow on Rosh Hashana and Yom Kippur when they say "Zokhreinu" ("Remember us") and "Mi Kamokha" ("Who is like You") [the insertions into the first blessing of the Amidah] need to straighten [themselves] up when they reach the end of the blessing. Gloss: And even though in [the blessing of] "Avot", one bows at the end of the blessing, nevertheless, one needs to straighten a little at the end of "Zokhreinu" so that it should be apparent that one is going back and bowing [again] because of the obligation [to bow at the end of the blessing of "Avot"] (His own opinion based on the Tur)

Se’if 3ג׳

הַכּוֹרֵעַ בִּוְכָל קוֹמָה לְפָנֶיךָ תִּשְׁתַּחֲוֶה, אוֹ בְּוּלְךָ אֲנַחְנוּ מוֹדִים, אוֹ בְּהוֹדָאָה דְּהַלֵּל וּבִרְכַּת הַמָּזוֹן, הֲרֵי זֶה מְגֻנֶּה. (פי' שֶׁאֵין לִכְרֹעַ אֶלָּא בְּמָקוֹם שֶׁתִּקְנוּ חֲכָמִים).

One who bows [when saying] "U'vechol Koma Lefanecha Tishtachaveh" ["and every upright one shall prostrate oneself before You"] or "U'lecha Anachnu Modim" ["and to You [alone] we give thanks"] [both from the "Nishmat Kol Chai" prayer], or [when saying] "Hoda'a" [Thanksgiving] in Hallel or Birkat Hamazon [The Blessings after a Meal], behold this is improper (meaning that one doesn't bow other than in a place that the Sages established).