"By the greatness of Your arm they were still as a stone." When Israel came out of the sea, Amalek gathered all the nations of the world and came and made war against Israel. And Moses prayed on behalf of the people, and in that hour they all fell silent as a stone. Therefore it is said, "By the greatness of Your arm they were still as a stone."
Amalek Gathers the Nations and Moses Prays Them Still
Yalkut Shimoni on Torah 251:16
בִּגְדֹל זְרוֹעֲךָ יִדְּמוּ כָּאָבֶן, כֵּיוָן שֶׁיָּצְאוּ יִשְׂרָאֵל מִן הַיָּם כִּנֵּס עֲמָלֵק אֶת כָּל אֻמּוֹת הָעוֹלָם וּבָא וְעָשָׂה מִלְחָמָה עִם יִשְׂרָאֵל, וְהִתְפַּלֵּל מֹשֶׁה בְּעַד הָעָם, בְּאוֹתָהּ שָׁעָה דָּמָמוּ כֻּלָּם כְּאֶבֶן, לְכַךְ נֶאֱמַר בִּגְדֹל זְרוֹעֲךָ יִדְּמוּ כָּאָבֶן.