אֵ֣ת כָּל־הַדָּבָ֗ר אֲשֶׁ֤ר אָנֹכִי֙ מְצַוֶּ֣ה אֶתְכֶ֔ם אֹת֥וֹ תִשְׁמְר֖וּ לַעֲשׂ֑וֹת לֹא־תֹסֵ֣ף עָלָ֔יו וְלֹ֥א תִגְרַ֖ע מִמֶּֽנּוּ׃ (פ)
Shulchan Arukh, Orach Chayim
A Kohen is not permitted to add anything on his own accord in addition to the three verses of Birkat Kohanim; and if he does add, he violates [the commandment of] do not add [to the Torah].