Midras Keluaran 14:4: Mekhilta DeRabbi Yishmael, Midrash Tanchuma, Pesikta Rabbati & Sifrei Bamidbar

(Exodus 14:4) "And I shall strengthen Pharaoh's heart": for it was divided, whether to pursue or not to pursue. "and I will be honored through (the downfall of) Pharaoh and all of his host": Pharaoh initiated the transgression, viz. (Exodus 1:22) "Every son that is born, into the Nile shall you throw him." Since he initiated the transgression, first he and then all of his host were drowned. Similarly, (Genesis 7:23) "And He blotted out every being … from man until beast, etc." The initiator was punished first. Similarly, (Ibid. 19:11) "And the men at the entrance of the house, etc." The initiators were punished first. Here, too, "and I will be honored through (the downfall of) Pharaoh, etc." The initiator was punished first. Now does this not follow a fortiori? If in the (L–rd's) measure of punishment, the lesser, the initiator of the transgression is punished first, then in his measure of reward, the greater, how much more so (is the converse true)!

This is the intent of (Exodus 14:4) "And Egypt will know that I am the L–rd." In the past, they did not know, but now they will know!

And I will harden Pharaoh’s heart (Exod. 14:4). This was said because his heart was undecided whether to pursue them or not. And I will get Me honor upon Pharaoh and upon all his hosts (Exod. 14:4). Upon Pharaoh is mentioned first because he was the first to sin, as it is said: And Pharaoh commanded all his people, saying (ibid. 1:22). Similarly, And the same day Pharaoh commanded the taskmasters (ibid. 5:6). Hence he was the first to be punished. Therefore, Surely now I have put forth My hand and smitten thee (ibid. 9:15) is followed by And thy people with pestilence.