אורח חיים קכ״חOrach Chayim 128

Se’if 1א׳

אֵין נְשִׂיאַת כַּפַּיִם בְּפָחוֹת מִי', וְהַכֹּהֲנִים מֵהַמִּנְיָן. וְאֵין לְזָר לִשָּׂא כַּפָּיו, אֲפִלּוּ עִם כֹּהֲנִים אֲחֵרִים (בְּפ' ב' דִּכְתֻבּוֹת דכ"ד דְּזָר עוֹבֵר בַּעֲשֵׂה, וְתוספ' פ' כָּל כִּתְבֵי לֹא יָדַע ר"י מָה אִסוּר יֵשׁ בְּזָר הָעוֹלֶה, וְאֶפְשָׁר דְּעִם כֹּהֲנִים אֲחֵרִים שָׁרֵי וְצָרִיךְ עִיּוּן).

Raising the Hands [to perform Birkat Kohanim - The Priestly Blessing] And Which Things Invalidate A Kohen. Containing 45 S'ifim: There is no "raising of the hands" [i.e. Birkat Kohanim] with less than ten [i.e. a quorum/minyan], and the Kohanim [who bless come from] the minyan [i.e. they are part of the initial minyan; not in addition to it]. A non-Kohen should not "raise the hands", even along with (others who are Kohanim) (Ketubot, ch. 2, daf 24, states that a non-Kohen violates a positive commandment) (but Tosafot in the chapter "Kol Kitvei" [Shabbat, ch. 16] states that the R"i does not know what prohibition there would be for a non-Kohen who ascends [for Birkat Kohanim], and it is possible that along with others who are Kohanim [it would be permitted]; but this requires further consideration).

Se’if 2ב׳

כָּל כֹּהֵן שֶׁאֵין בּוֹ אֶחָד מֵהַדְּבָרִים הַמְעַכְּבִים, אִם אֵינוֹ עוֹלֶה לַדּוּכָן אַף עַל פִּי שֶׁבִּטֵּל מִצְוַת עֲשֵׂה אַחַת, הֲרֵי זֶה כְּעוֹבֵר בְּג' עֲשֵׂה אִם הָיָה בְּבֵית הַכְּנֶסֶת כְּשֶׁקָּרְאוּ כֹּהֲנִים, אוֹ אִם אָמְרוּ לוֹ לַעֲלוֹת אוֹ לִטֹּל יָדָיו.

Any Kohen who does not have one of the things that prevent [him from performing Birkat Kohanim] — if he does not ascend to the platform, even though he has [only] forfeited one positive commandment, it is as if he has violated three positive commandments if he was in the synagogue when they called "Kohanim" or if they told him to go up or to wash his hands.

Se’if 3ג׳

אִם עָלָה פַּעַם אַחַת בְּיוֹם זֶה, שׁוּב אֵינוֹ עוֹבֵר, אֲפִלּוּ אָמְרוּ לוֹ: עֲלֵה.

If he had gone up once [already] that day, he would not be violating [the positive commandment if he did not go up subsequent times], even if they told him, "Go up."