Old Testament readings use the Septuagint , the Scripture the apostles quoted. Masoretic numbering shown for reference.Learn why
LXX

1 Samuel 15:18

And the Lord sent thee on a journey, and said to thee, Go, and utterly destroy: thou shalt slay the sinners against me, [even] the Amalekites; and thou shalt war against them until thou have consumed them.

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