כה תברכו את בני ישראל אמור להם (נשא ו:כג)

Rashi says on this passuk: אמור להם – כמו שמור וזכור – talk to them like you spoke to them about Shabbos.

One Erev Shabbos, there was no fish in the home of Reb Mendele of Riminov Zt”l for the Shabbos meals.  The Rebbe’s shamash (assistant), who was then known as Reb Hersh Meshuras, went to the market to find fish for Shabbos.  His search was a complete failure, as he could not find any fish at all.  He returned to Reb Mendele, saying there was no fish in the Erev Shabbos market.

Reb Mendele told him to prepare a pot with water, place an onion inside, and put it on the fire. Reb Mendele insisted that while Reb Hersh is busy preparing, he should constantly say, “Lekovod Shabbos Kodesh” (in honor of the holy Shabbos).

When it was almost the z’man (time) of Shabbos, a chassid came into the house with fish, which was promptly placed into the pot, and there was plenty of fish for the Shabbos meals.  Reb Mendele explained that the holiness of Shabbos brings down good things from heaven.  To be the recipient of the good, one has to verbally say with his mouth that he is preparing “Lekovod Shabbos Kodesh.”  Therefore, when Reb Hersh kept saying “Lekovod Shabbos Kodesh,” the fish had to be ‘heaven sent.’

Reb Hersh Shamash added to his Rebbe’s words that the same is true for Birchas Kohanim — the blessing of the Kohanim: Hashem blesses the Yidden with all the brachos, and for the brachos to come from heaven, we need the Kohanim to verbalize the brachos.

This is what Rashi alludes to: “אמור להם – כמו שמור וזכור” – these words are about Shabbos Kodesh. Just like with Shabbos, one has to verbalize “Lekovod Shabbos Kodesh” as it brings down good things from above, so it is with Birchas Kohanim. When the Kohanim say the blessings, the effect of their saying the words brings blessing upon the Yidden.