"And the LORD said to Moses, Go down, warn the people" (Exodus 19:21): testify against the people, "lest they break through," lest they press forward to see. "And many of them fall": this teaches that permission was given to the agents [of destruction] to strike. Another interpretation: even if many of them, that is, even one great one among them, should fall, he is reckoned as all of them. This teaches that even one [lost] is held back against all of them. A single individual who falls from among them is reckoned as equal to the whole of creation, as it is said, "For the eye of man is toward the LORD" and so forth (Zechariah 9:1).
Warn the People Not to Break Through to See
Yalkut Shimoni on Torah 284:2
(שמות יט כא) וַיֹּאמֶר ה' אֶל מֹשֶׁה רֵד הָעֵד בָּעָם, הַסְהֵד בָּעָם פֶּן יְהֶרְסוּ, שֶׁמָּא יִדָּחֲקוּ לִרְאוֹת. וְנָפַל מִמֶּנּוּ רָב, מְלַמֵּד שֶׁנִּתְּנָה רְשׁוּת לַשְּׁלוּחִין לְחַבֵּל, דָּבָר אַחֵר אֲפִילוּ רָב אֶחָד מֵהֶן שֶׁיִּפֹּל הֲרֵי הוּא כְּכֻלָּן, מְלַמֵּד שֶׁאַף אֶחָד מְעַכֵּב עַל יְדֵי כֻּלָּם, יְחִידִי שֶׁיִּפֹּל מֵהֶן הֲרֵי הוּא כְּנֶגֶד כָּל מַעֲשֵׂה בְּרֵאשִׁית, שֶׁנֶּאֱמַר (זכריה ט, א) "כִּי לַה' עֵין אָדָם" וְגוֹ'.