Midrash sa Exodo 8:17: Shemot Rabbah, Midrash Tanchuma & Aggadat Bereshit

Another interpretation: “Rise early in the morning” – the Holy One blessed be He said to Moses: ‘This wicked one hardened his heart during the first three. The fourth plague is more severe than the previous ones; go and forewarn him that he should let My people go so the plague will not come upon him.’
“For, if you do not let My people go, behold, I will send swarms upon you, upon your servants, upon your people, and into your houses; and the houses of the Egyptians will be full of swarms, and also the ground on which they are. I will distinguish on that day the land of Goshen, in which My people dwell, that no swarms will be there; so that you will know that I am the Lord in the midst of the earth” (Exodus 8:17–18).
“For, if you do not let My people go, behold, I will send…upon you…” – from where did they come upon them? Some say from above and some say from below. Rabbi Akiva says: From above and from below. Rabbi Shimon ben Lakish said: The Holy One blessed be He said to them: ‘You made many multitudes over My children; I, too, will make upon you many multitudes of birds of the heavens and beasts of the land,’ as it is stated: “Behold, I will send swarms upon you” – beasts and birds intermingled. However, in the land of Goshen, in which Israel was residing, there were no swarms, as it is stated: “I will distinguish on that day the land of Goshen.” Rabbi Ami says: It is like a person who says to another: ‘So-and-so will not be included in this penalty, as his patron, so-and-so, is standing over him.’³He is protecting them.
“I will place a division between My people and your people, tomorrow this sign will be” (Exodus 8:19).
“I will place a division between My people [and your people]” – this teaches that Israel was fit to be afflicted with this plague, but the Holy One blessed be He placed the Egyptians as their ransom. In the future, too, the Holy One blessed be He will bring (ancient)⁴This word, which appears in some editions, was added by the non-Jewish censor. idolaters and cast them into Gehenna in place of Israel, as it is stated: “For I am the Lord your God, Holy One of Israel, your Savior; I gave Egypt as your ransom, Kush and Seva in your stead” (Isaiah 43:3).

The verses My soul hath them still in remembrance and is bowed down within me (Lam. 3:20) and It may be that the Lord will look on mine eyes (II Sam. 16:12) were amended by the men of the Great Synagogue. They are called scribes because they counted every letter in the Torah and interpreted it.¹²Sofer (“scribe”) is derived from safar (“to count”). Thus they amended And lo, they put the branch unto My nose (Exod. 8:17) to read their nose. Likewise, they altered the verse we are discussing Surely he that toucheth you toucheth the apple of My eye (ibid.) to read His eye. This was to teach us that everyone who rises up against Israel is considered as rising up against the Shekhinah. Hence it says: Curse ye bitterly the inhabitants thereof, because they came not to the help of the Lord (Judg. 5:23).

[2] Another interpretation: When it was time to present the meal offering (1 Kings 18:36) David said, "My prayer, O Lord, is at an opportune time" (Psalm 69:14). God has appointed a time for everything except prayer. When should one pray in order to be answered? As it says, "For every desire there is a time and a judgment" (Ecclesiastes 8:6). If a person knew when to pray and be answered, he would only pray on that day. Therefore, God said, "I will not let you know when you will be answered, so that you will pray at all times" as it says, "Trust in Him at all times" (Psalm 62:9). Even the forefathers of the world were sometimes answered and sometimes not. Moses, the great leader of the world, prayed and was answered, as it says, "And the Lord did as Moses asked" (Exodus 8:17). And so it says, "I have forgiven you as you asked" (Numbers 14:20). But sometimes, even when he prayed, he was not answered, as it says, "But the Lord was angry with me because of you" (Deuteronomy 3:26). David was answered at all times, as it says, "I sought the Lord and He answered me" (Psalm 34:5), and also, "And He answered him from heaven with fire" (1 Chronicles 21:26). When was he not answered? When the child was sick and he fasted, lay on the ground, and begged for mercy, but in the end the child died. When is the opportune time for everyone? As it says, "At an opportune time I answered you" (Isaiah 49:8). David said to God, "Master of the universe, every hour that I pray, make it an opportune time for me to be answered" as it says, "My prayer, O Lord, is at an opportune time" (Psalm 69:14). When is the opportune time? It is the afternoon prayer, as it says, "And it came to pass at the time of the evening offering" (1 Kings 18:36).