Rashi on Numbers 23:20/רש"י על במדבר כ״ג:כ׳

1הנה ברך לקחתי BEHOLD, I HAVE RECEIVED COMMANDMENT TO BLESS — You ask me derisively, (v. 17) “What hath the Lord spoken”? I will tell you: I have received an order from Him to bless them.

2וברך ולא אשיבנה AND HE HAS BLESSED AND I CANNOT REVERSE IT — “He” has blessed them, and “I” cannot reverse this blessing.

3וברך — This is equivalent to וּבִרֵּךְ. This is the rule of the letter ר, as in (Psalms 74:18): “the enemy blasphemeth”, חֵרֵף like חִרֵּף; and similarly (Psalms 10:3): [ובצע בֵּרֵךְ [נאץ ה׳ (where בֵּרֵךְ is for בִּרֵּךְ), which means, He who praises and blesses the robber (בצע) and says, ‘Do not be afraid, for you will not be punished; peace will be unto you’, he angers (נאץ) God thereby (but see Rashi on this verse where another translation is given). — One cannot say that ברך is a noun, for if it were, it would be voweled in both syllables with Short Patach (our Segol), and its accent would be on the first syllable (בֶּרָךְ) because it is a verb in the Piel conjugation, it is voweled with Short Kametz (our Tzéré) on both syllables, and its accent is on the last syllable.