שולחן ערוך אורח חיים · סימן קכ״א

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The Laws of "Modim". Containing 3 S'ifim: We bow in "Modim" ["We are thankful"] at the beginning [of it] and at the end.

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One who says "Modim Modim", we silence [that person].

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An individual does not say "Birkat Kohanim" ["The Priestly Blessing"]. Gloss: And this is the principle, and it appears to me that [people should] practice like this. But the the widespread custom is not like this, rather even an individual says it any time it is appropriate to "spread the hands" [i.e. to say Birkat Kohanim], but this does not appear [correct to me]. (Beit Yosef in the name of Manhig [Sefer haManhig by Rabbi Avraham ben Natan, 13th century Provence])