Rashi on Exodus 8:27/רש"י על שמות ח׳:כ״ז

1ויסר הערב AND HE REMOVED THE SWARM OF ANIMALS — He merely removed them and they did not die as the frogs had died; for had they died, they (the Egyptians) could have made some use of their skins (Exodus Rabbah 11:3).