בַּעֲשׂוֹתְךָ֥ נוֹרָא֖וֹת לֹ֣א נְקַוֶּ֑ה יָרַ֕דְתָּ מִפָּנֶ֖יךָ הָרִ֥ים נָזֹֽלּוּ׃
when Thou didst tremendous things which we looked not for— Oh that Thou wouldest come down, that the mountains might quake at Thy presence!—
Rashi on Isaiah
When You performed against the Egyptians and against all the adversaries awesome deeds, that we did not hope You would perform all those awesome deeds, for we were unworthy of them.
Rashi on Isaiah
You descended to Mount Sinai, then mountains dripped from before You. In this manner, Dunash son of Labrat explained it.