Wyjścia 14:7 Midrasz: Mekhilta DeRabbi Yishmael, Eikhah Rabbah & Midrash Tanchuma

And thus do you find with the Egyptians, that with what they vaunted themselves, He exacted punishment of them, (Exodus 14:7) "And he (Pharaoh) took six hundred chariots and all (the other) chariots of Egypt," and it is written (Ibid. 15:4) "The chariots of Pharaoh and his host He cast into the sea, etc."

(Exodus 15:1) "A horse and its rider He has cast into the sea": Now was there only one horse and one chariot? Is it not written (Ibid. 14:7) "And he took six hundred choice chariots"? — When Israel does the will of the L–rd, its foes oppose it, as it were, with one horse and its rider. Similarly, (Devarim 20:1) "When you go out to war against your enemy and you see horse and chariot." Now was there only one horse and one chariot? — When Israel does the will of the L–rd, etc."

(Exodus 14:7) "And he took six hundred choice chariots": Whence came the horses required for the chariots? If you would say, from Egypt, is it not written (re the plague of pestilence, Ibid. 9:6) "and all the cattle of Egypt died"? And if you would say, from Pharaoh, is it not written (Ibid. 3) "Behold, the hand of the L–rd is in your cattle in the field, in the horses, etc."? And if you say, from Israel, is it not written (Ibid. 10:26) "And our cattle, too, will go with us; not a hoof will remain"? Whence, then, did they come? From those (of his servants) who feared the word of the L–rd (viz. 9:20) and drove his horses from the field into the houses. We find, then, that the cattle driven off by those who feared the word of the L–rd proved to be an impediment to Israel — whence R. Shimon says: "the best of the gentiles — kill! The best of the serpents — crush its head!"