Exodus 17:12

וידי משה כבדים BUT MOSES HANDS BECAME HEAVY — Because he had shown himself remiss in the duty that was his, of waging war himself against Amalek, and had appointed another in his stead, his hands became heavy (cf. (Mekhilta). ויקחו AND THEY TOOK — Aaron and Hur took — אבן וישימו תחתיו A STONE AND THEY PUT IT UNDER HIM [AND HE SAT THEREON] — but he did not seat himself upon a cushion or a pillow, for he said, “Israel is in tribulation; I will be in tribulation together with them”. (Taanit 11a). ויהי ידיו אמונה (lit., and he was, his hands faithfulness) — and Moses was in the condition that his hands were in faithfulness — spread forth to heaven in a confident and firm prayer. עד בא השמש UNTIL THE GOING DOWN OF THE SUN — Because the Amalekites had calculated the hours by astrology, as to which hour they would prove victorious, Moses held back the sun against them and brought the hours into confusion (Midrash Tanchuma, Beshalach 28).

וִידֵ֤י מֹשֶׁה֙ כְּבֵדִ֔ים וַיִּקְחוּ־אֶ֛בֶן וַיָּשִׂ֥ימוּ תַחְתָּ֖יו וַיֵּ֣שֶׁב עָלֶ֑יהָ וְאַהֲרֹ֨ן וְח֜וּר תָּֽמְכ֣וּ בְיָדָ֗יו מִזֶּ֤ה אֶחָד֙ וּמִזֶּ֣ה אֶחָ֔ד וַיְהִ֥י יָדָ֛יו אֱמוּנָ֖ה עַד־בֹּ֥א הַשָּֽׁמֶשׁ׃

But Moshe’s hands were heavy; and they took a stone, and put it under him, and he sat on it; and Aharon and Ḥur supported his hands, the one on the one side, and the other on the other side; and his hands were steady until the going down of the sun.