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How did Hashem reward David for his virtue?
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Mahari Kara: The second time David passed up an opportunity to kill Sha'ul, he said "Hashem should return the Tzedakah" (Shmuel I, 26:23); Malbim (26:23)
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Shmuel I, 24:19: [The first time he refrained, Sha'ul said] "Hashem should pay good to you in place of today, that you did to me." Here is past tense; David points out that Hashem fulfilled this. (PF)