Rashi on Exodus 17:12/רש"י על שמות י״ז:י״ב

1וידי 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).

2ויקחו AND THEY TOOK — Aaron and Hur took —

3אבן 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).

4ויהי 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.

5עד 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).