First, he lifted up both hands to heaven. For in the Hebrew a singular is put for a plural, as not only our Latin [translator] shows, but also the Septuagint, and the very reasoning which is subjoined in the plural: “But Moses’s hands were heavy.” [That] he also stretched out these same [hands], Origen denies (homily 11 on Exodus). Yet the contrary of this is persuaded not only by the common depiction [in art], but also by the ancient custom of the Jews, and by the very supporting of the arms afterward made. For since on one side stood Aaron, on the other Hur, it appears [that] the arms and hands were outstretched [spread]. And so Theodoret feels (on Exodus, question 34), and Tertullian (book 3 against Marcion): “And he stretched out [his] hands, that by a sign he might conquer the enemy.”
Secondly, in one of [his] hands he held the rod of God. For so he had promised he would do (v. 9). And about the names, matter, and dignity of this rod I have spoken [on] Exodus 7. Thirdly, to this gesture of the body, and to the ceremony, he joined ardent prayers. For that is both gathered from the common opinion of all, and from these very ceremonies. For that stretching-out of the hands, and extending of the rod, was undertaken on account of the prayers themselves. Fourthly, he did these [things] at first standing. For in v. 9 he says “I will stand,” where the Septuagint used the prophetic past [tense] — which notes a thing certainly to come — ἕστηκα for στήσομαι, as it is also in the Hebrew. Fifthly, in the morning Moses begins that prayer, and continued it the whole day — as, rightly, in the Elucidatory annotations, Hugh of St. Victor says — until the setting of the sun. For the end [terminus] here is expressly set in the text; but that morning beginning is gathered — first indeed from this, that Moses had said, in the same v. 9, “To-morrow I will stand.” For since on that day the battle was to be joined, and he had entrusted all the other [matters] to Joshua, nothing else remained but that he should do this one [thing]: betake himself at once to prayer, get ahead of [be at his post before] the Israelite warriors themselves as they went out from the camp, [and] display that rod as, so to speak, a most certain sign of victory — and thus make huge courage for all. Then [it] is also gathered from the other terminus. For [he] who says [that] anyone did anything up to the setting of the sun, if he does not add noon, or another time at which he began [it], wishes it to be signified [that] he began from the rising of the same sun, or in the early morning. And so Josephus narrates, at the cited place. Sixthly, hence indeed it follows [that] the same [Moses] persisted fasting the whole day too. The chosen soldiers took care, as is the custom, [to refresh] their bodies with rest, and with some food, that [they might be] more upright [ready] for the fight, and more tolerant of dust and of labor — especially [when it is] long and bloody.
(col. 71)
But Moses [for his part] prepared [his] soul; and, that [his] vows might be the more penetrating into heaven, he joined fasting to these. And that he did not refrain from tears, his easily-blazing and most humble religion and attention of soul readily persuades me. Certainly [that] he bent [his] knees to the ground, and prostrated himself prone upon [his] face at times, R. Eliezer writes (ch. 44) — as we shall hear toward the end of this question. Nay, as Josephus says at the cited place, on the night before the battle he kept vigil, and taught Joshua himself how he should array the army; but, as Philo [says] at the cited place, he purified himself with solemn lustrations. And thus chaste, pure, and holy, he ascended the mountain — although still, that very morning, he had excellently confirmed [encouraged] that armed multitude with words, as Josephus adds.
But while he stood there and prayed in the manner just said, from too long a delay [his] knees began to grow weary, [his] hands to droop, and [his] whole aged body to grow faint. Yet the excellent suppliant did not desist from what he had begun, nor did the old man begin to give way at once, or to sit; much less to ask for food, or a cushion; but against [his] faintness, and the weakness of his body, he strove with all [his] strength. He gathered himself up again as he was slipping, again raised [his] hands, again lifted the rod. And this he did so often, and so long, until his strength altogether failed him, and [there was] nothing more he could [do]. Eighthly, then indeed he willed neither to make an end of praying, nor to depart from the place, but betook himself to another mode of praying — about which now [a word]. Origen, toward the end of homily 6 on Leviticus: “Moses,” he says, “does not go to the wars, does not fight against the enemies. But what does he do? He prays. And as long as he prays, his people conquers. If he shall have relaxed and let down [his] hands, his people is conquered and put to flight. Let the priest of the Church, therefore, pray unceasingly, that the people may conquer — [the people] who [are] under him — and [may conquer] the invisible enemies, the Amalekites, who are demons, assailing those who wish to live piously in Christ.” And [it is] a splendid and most useful example for all people, when in good matters the Princes, Pontiffs, [and] Ecclesiastics lead the way. “All,” says R. Eliezer (in the cited ch. 44), “Israel — or, all the Israelites — stood outside their tents, and saw Moses praying with bended knees: and therefore they too bent their knees. They saw him falling forward upon his face: they too fell forward upon their faces. They saw him lifting his hands to the heavens: they too lifted theirs. Whence it must be said [that], the head of the congregation praying — [he] who is, as it were, its Apostle and intermediary with God — all the people is wont to follow him, and to answer ‘Amen.’” Whence St. Jerome (book 2 against Jovinian): “In Exodus he says [that] against Amalek [the battle] was fought by the prayer of Moses, and of the whole people, fasting until evening.”