TT #6 · Parashas Va’eschanan · First Aliyah · Passuk 4:2
You shall not add to the word that I command you, nor shall you diminish from it, to keep the commandments of Hashem your God that I command you.
Torah Temimah points to the baraisa that a kohen may not invent additional berachos of his own when ascending the duchan (and parallel discussions of the prohibition of adding to the Torah).
See also the parallel in Re’eh 13:1.
English: It was taught: From where do we know that a kohen who ascends the duchan may not say, “Since the Torah gave me permission to bless Israel, I will add a berachah of my own…”? From the verse “You shall not add to the word.”
Custom (application of the lo-tosifu prohibition to the birkas kohanim).
One may not add personal inventions to the fixed forms of mitzvah or berachah that the Torah has already prescribed; the duchan blessing is a concrete example.