Midrash Tanchuma, Shemot
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דָּבָר אַחֵר, יַרְעֵם אֵל בְּקוֹלוֹ נִפְלָאוֹת. אֵימָתַי עָשָׂה הַקָּדוֹשׁ בָּרוּךְ הוּא בְּקוֹלוֹ נִפְלָאוֹת? בְּשָׁעָה שֶׁבִּקֵּשׁ לִשְׁלֹחַ מֹשֶׁה אֶל פַּרְעֹה לִגְאֹל אֶת יִשְׂרָאֵל מִמִּצְרַיִם, וְהָיָה מֹשֶׁה בְּמִדְיָן, שֶׁהָיָה מִתְיָרֵא מִפְּנֵי פַּרְעֹה. כֵּיוָן שֶׁנִּגְלָה הַקָּדוֹשׁ בָּרוּךְ הוּא עָלָיו בְּמִדְיָן וְאָמַר לוֹ לֵךְ שֻׁב מִצְרָיִם, נֶחֱלַק הַדִּבּוּר לִשְׁנֵי קוֹלוֹת וְנַעֲשָׂה דּוּ פַּרְצוּפִין, וְהָיָה מֹשֶׁה שׁוֹמֵעַ בְּמִדְיָן לֵךְ שֻׁב מִצְרָיִם, וְאַהֲרֹן שׁוֹמֵעַ לֵךְ לִקְרַאת מֹשֶׁה. וּמִי שֶׁהָיוּ בָּאֶמְצַע, לֹא הָיוּ שׁוֹמְעִין כְּלוּם, הֱוֵי יַרְעֵם אֵל בְּקוֹלוֹ נִפְלָאוֹת.
Another explanation of God thundereth marvelously with his voice (Job 37:5). When did the Holy One, blessed be He, perform these miracles with His voice? He did so at the time he wished to send Moses to Pharaoh to redeem the Israelites from Egypt. When Moses was in Midian, he was afraid to go to Pharaoh, so the Lord revealed Himself him and told him: Go, return into Egypt (Exod. 4:19). These words divided themselves into two distinct sounds and took two different forms. Moses heard the voice in Midian saying: Go return into Egypt, and Aaron heard it saying: Go, meet Moses (ibid., v. 27), while those in between heard nothing at all. Hence, God thundereth marvelously, etc.