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

Rashi comments: אמור להם – כמו זכור.שמור – tell them, like what it says about the Shabbos commandment – to remember and guard.

In the home of the Rebbe, Reb Mendel of Riminov zt’’l, there wasn’t any fish on Erev Shabbos. The Rebbe’s shamesh, R’ Hersh Meshuras, searched the market for fish l’kovad Shabbos but found no fish.

He returned from the market and told the Rebbe that there was no fish to be found for Shabbos.  Rebbe Mendele told him to boil some water with onions and salt and place the fish inside, and while making the preparations, he should say, “l’kovad Shabbos Kodesh!”

Reb Hersh obeyed his Rebbe.  

Suddenly, a few minutes before Shabbos, a chassid brought fish to the Rebbe. Rebbe Mendele explained, “Evidently, the holiness of Shabbos brings down השפעות – (good effects).  To be worthy of these things, one has to say with his mouth that you’re preparing l’kovad Shabbos.

Rebנק Mendele added that the same is with Birchas Kohanim.  Hashem blesses the Yidden with all brachas, but for the brochos to take effect, the kohanim must recite it by mouth.  This is what Rashi alludes with “אמור להם כמו זכור שמור” – these words are said about the Shabbos Kodesh.  And just like one has to mouth the words לכבוד שבת קודש to bring down its good effects from Above, so too with Birchas Kohanim.