Midras Keluaran 8:24: Midrash Tanchuma, Mekhilta DeRabbi Yishmael & Shemot Rabbah

And it came to pass, when Pharaoh had let the people go (Exod. 13:17). The word let go (shallah) indicates “to escort,” as in the verse Abraham went with them to bring them (leshalleham) on the way (Gen. 18:16). Similarly it states; And Isaac sent them away (vaye-shallehem), and they departed from him in peace (ibid. 26:31). Why does this scriptural portion open with this verse? Because the mouth (i.e., Pharaoh) that had declared: And moreover, I will not let Israel go (Exod. 5:2), now retracted and said: I will let you go (ibid. 8:24). This explains why this portion of Scripture begins with the words When Pharaoh had let the people go. What was his reward for doing so? Thou shalt not abhor an Egyptian (Deut. 23:8), for the mouth that had said: I know not the Lord (Exod. 5:2) reversed itself and declared: The Lord is righteous (ibid. 9:27). What was his reward for saying this? He gave them a burial place, as it is said: Thou stretchest out Thy right hand—the earth swallowed them (ibid. 15:12).

(Exodus 13:17) "And it was, when G–d sent ("shalach") the people": "sending" in all places is accompaniment, viz. (Genesis 18:16) "And Abraham went with them to send them," (Ibid. 26:31) "And Israel sent them." The mouth (of Pharaoh) that said (Exodus 5:2) "Israel, too, I will not send," it is that (mouth) which said (Ibid. 10:10) "I will send you and your children." How was he rewarded for this? (Devarim 23:8) "You shall not abominate an Egyptian." The mouth which said (Exodus 5:2) "I do not know the L–rd," it is that (mouth) which said (Ibid. 14:25) "I will flee from before Israel, for the L–rd wars for them against the Egyptians." How was he rewarded for this? (Isaiah 19:19) "On that day there will be an altar to the L–rd in the midst of the land of Egypt and a pillar at its border to the L–rd." The mouth which said (Exodus 5:2) "Who is the L–rd that I should hearken to His voice," it is that mouth which said (Ibid. 9:27) "the L–rd is the Tzaddik, and I and my people are the wicked" — wherefore He gave them a place for burial, as it is written (Ibid. 15:12) "You inclined Your right hand — the earth swallowed them up."

“The Lord did so; and heavy swarms came into the house of Pharaoh, and into the house of his servants; and throughout the land of Egypt the land was ruined due to the swarms” (Exodus 8:20).
“The Lord did so; and heavy swarms came into the house of Pharaoh” – he was afflicted first, as he began with the wicked counsel first, as it is stated: “Pharaoh commanded…” (Exodus 1:22), and then, “the house of his servants.”
“Pharaoh called for Moses and Aaron, and said: Go sacrifice to your God in the land. Moses said: It is not proper to do so, as we will sacrifice the abomination of the Egyptians to the Lord our God; behold, were we to sacrifice the abomination of the Egyptians before their eyes, would they not stone us?” (Exodus 8:21–22).
“Pharaoh called.… Moses said: It is not proper to do so” – because the Egyptians prostrate themselves to these animals as a god.
“We will go on a three-day journey into the wilderness, and sacrifice to the Lord our God, as He will command us” (Exodus 8:23).
“We will go on a three-day journey into the wilderness” – in order to mislead them.
“Pharaoh said: I will let you go, and you will sacrifice to the Lord your God in the wilderness; just do not go very far; entreat for me. Moses said: Behold, I am leaving you, and I will entreat the Lord, and the swarms will depart from Pharaoh, from his servants, and from his people, tomorrow; only let Pharaoh not continue to be deceitful by not letting the people go to sacrifice to the Lord” (Exodus 8:24–25).
“Pharaoh said: I will let you go.… Moses said: Behold, I am leaving you” – what is “behold”? The prayer will be immediate, so that the swarm will depart from you tomorrow. That is what is written: “The swarms will depart…[tomorrow].”
“Moses left Pharaoh, and entreated the Lord. The Lord did according to the word of Moses; and He removed the swarms from Pharaoh, from his servants, and from his people; not one remained” (Exodus 8:26–27).
“Moses left.… The Lord did according to the word of Moses; and He removed the swarms” – why did He bring swarms upon them? It is because they would say to Israel: ‘Go out and bring us bears, lions, and leopards,’ in order to torment them; therefore, He brought upon them swarms of beasts; this is the statement of Rabbi Yehuda. Rabbi Neḥemya said: [They were] species of hornets and mosquitoes. The statement of Rabbi Yehuda appears correct, as regarding the frogs it is written: "The frogs died” (Exodus 8:9), because there was no benefit from their skins; however, regarding the swarms there was [potential] benefit from their hides; therefore, “not one remained.” Had they been hornets or mosquitoes they could have been left to reek.