Exodus 17:11 שמות י״ז:י״א
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וכאשר יניח ידו. על דרך הפשט כי לא יוכל להרים ידו כל היום וכאשר יניחם באונס גבר עמלק על כן צוה לאהרן וחור שיתמכו בידיו. ובמדרש וכי משה היה עושה שיתגבר עמלק אלא אסור לאדם לשהות כפיו פרושות השמימה ג' שעות. ור' יוסף קמחי פי' שלא הניח אלא בשביל שידע שכאשר יניח ידו וגבר עמלק וידע שידיו כבדים ולא יוכל להרימם כל היום לקח עמו אהרן וחור שיתמכו ידיו. וי"מ לפי הפשט והיה כאשר ירים ידיו עם המטה וגבר ישראל שהמטה היה סימן שלהם כמו שעושין דגל במלחמה שלא יפזרו אנשי המלחמה ולכן כתיב ויקחו אבן וישימו תחתיו וישב עליה כדי שיהי' במקום אחד גבוה ויראו כולם הנס שלהם ועל כן קרא משה למזבח ה' ניסי לומר לא במטי שהרימותי לנס גברו ישראל אלא ה' נסי ודגלי:

וכאשר יניח ידו, “and whenever he allowed his hand to rest,” according to the plain meaning of the text it was simply physically impossible for Moses to maintain a posture with raised hands all day long. Whenever he was too tired to maintain his posture, the battle tended to favour the Amalekites. This is why he commanded Aaron and Chur to assist him in maintaining the posture with his hands raised in support of his prayer. According to the Midrash, where the question is asked rhetorically whether Moses’ hands determined the outcome of the battle, the answer given is that Moses was not suffering from physical fatigue, but that it is forbidden for any man to keep his hands outstretched in prayer for three hours or more without a break. [It is an aspect of עיון תפילה, praying “insistently,” something most unbecoming for man in his relation to his Creator. Ed.] Rabbi Joseph Kimchi explains that the words וכאשר יניח ידו do not describe something that Moses actually did, but these words describe that Moses knew that he could not afford to allow his hands to rest, else Amalek would prove victor, and that in order to forestall such an eventuality, he took Aaron and Chur with him from the start to help him support his hands. Some commentators understand Moses’ raising his hands as referring to the hands with which he was holding his staff as a sign of encouragement to the people. Moses’ staff meant for the people what the flag means to gentile troops. Generally, the purpose of the flag is to serve as a point around which the troops position themselves, to prevent being scattered by opposing forces. The description of Moses being placed in an elevated position even while seated, is to signify that Moses performed the function that a flag normally performs in a battle. This explains why Moses called the altar he built after the battle ה' ניסי. He did not want the people to think that their victory had been due to the visibility of Moses’ staff during the battle, but that it was exclusively due to the help of Hashem, without which the staff would not have proven effective at all. Israel’s “flag” is its faith in Hashem.