"And Aaron and Hur supported his hands." From here you learn that a prayer-leader is forbidden to pray unless two stand, one at his right and one at his left. They said: Israel went out from their tents and saw Moses kneeling upon his knees, and they knelt upon their knees; and falling upon his face to the ground, and they fell upon their faces to the ground. Just as the prayer-leader prays, so the people answer. "And his hands were faithfulness until the going down of the sun." Now until that day had the sun not set? Rather, from here that the sun stood still for him. This is what Scripture says, "Day to day pours forth speech" (Psalms 19:3) - this is the day of Moses, which announced the day of Joshua. He said: May it be Your will that the sun stand still for you as it stood still for me. Therefore it is said, "And place it in the ears of Joshua" (Exodus 17:14).
Israel Bowed as Moses Bowed and the Sun Stood Still for Him
Yalkut Shimoni on Torah 265:3
וְאַהֲרֹן וְחוּר תָּמְכוּ בְיָדָיו. מִכַּאן אַתָּה לָמֵד שֶׁשְּׁלִיחַ צִבּוּר אָסוּר לְהִתְפַּלֵּל אִם אֵין שְׁנַיִם עוֹמְדִים אֶחָד מִימִינוֹ וְאֶחָד מִשְּׂמֹאלוֹ. אָמְרוּ, יִשְׂרָאֵל יָצְאוּ חוּץ לְאָהֳלֵיהֶם וְיִרְאוּ אֶת מֹשֶׁה כּוֹרֵעַ עַל בִּרְכָּיו, וְהֵם כּוֹרְעִין עַל בִּרְכֵּיהֶם. וְנוֹפֵל עַל פָּנָיו אַרְצָה, וְהֵן נוֹפְלִין עַל פְּנֵיהֶן אַרְצָה. כְּשֵׁם שֶׁשְּׁלִיחַ צִבּוּר מִתְפַּלֵּל כָּךְ הָעָם עוֹנִים. וַיְהִי יָדָיו אֱמוּנָה עַד בֹּא הַשָּׁמֶשׁ. וְכִי עַד אוֹתוֹ יוֹם לֹא בָּא הַשֶּׁמֶשׁ. אֶלָּא מִכָּאן שֶׁעָמְדָה לוֹ הַשֶּׁמֶשׁ, זֶהוּ שֶׁאָמַר הַכָּתוּב "יוֹם לְיוֹם יַבִּיעַ אֹמֶר", זֶה יוֹמוֹ שֶׁל מֹשֶׁה שֶׁבִּשֵּׂר יוֹמוֹ שֶׁל יְהוֹשֻׁעַ, אָמַר, יְהִי רָצוֹן שֶׁתַּעֲמוֹד לְךָ חַמָּה כְּשֵׁם שֶׁעָמְדָה לִי, לְכַךְ נֶאֱמַר (להלן פסוק יד) וְשִׂים בְּאָזְנֵי יְהוֹשֻׁעַ.