Rashi on Exodus 17:13/רש"י על שמות י״ז:י״ג

1ויחלש יהושע AND JOSHUA DISCOMFITED [AMALEK] (more lit., he weakened Amalek) — he cut off the heads of his mighty men and left only the weak amongst them, and he did not kill all of them (leaving the weak men alive: thus Amalek was made weak, and powerless for further mischief). From this we may learn that they acted according to the expressed pronouncement of the Shechina (otherwise they would, in the stress of battle, not have so discriminated) (Mekhilta).