Exodus 4:23

ואמר אליך AND I HAVE SAID UNTO THEE, as a messenger of the Omnipresent God, שלח את בני LET MY SON GO, הנה אנכי הרג וגו׳ BEHOLD I WILL SLAY etc. — This, it is true, was the last of the plagues, but it was by mention of it that God gave him the first warning regarding the plagues that would come upon him, because this was the severest of them all. This is what Scripture means by, (Job 36:22) “Behold God is mighty in His power”, therefore “Who is a teacher like unto Him?” — This signifies: A human being who intends to avenge himself upon his fellow keeps his plans secret in order that he may not seek means of escape. But the Holy One, blessed be He, is so mighty in His power that there is no possibility of escape from His hand except by returning to Him in penitence. Just on this account He teaches him (informs him what punishment will befall him) and so warns him to return to Him (Exodus Rabbah 9:9).

וָאֹמַ֣ר אֵלֶ֗יךָ שַׁלַּ֤ח אֶת־בְּנִי֙ וְיַֽעַבְדֵ֔נִי וַתְּמָאֵ֖ן לְשַׁלְּח֑וֹ הִנֵּה֙ אָנֹכִ֣י הֹרֵ֔ג אֶת־בִּנְךָ֖ בְּכֹרֶֽךָ׃

and I say to thee, Let my son go, that he may serve me: and if thou refuse to let him go, behold, I will slay thy son, thy firstborn.