וָאֹמַ֣ר אֵלֶ֗יךָ שַׁלַּ֤ח אֶת־בְּנִי֙ וְיַֽעַבְדֵ֔נִי וַתְּמָאֵ֖ן לְשַׁלְּח֑וֹ הִנֵּה֙ אָנֹכִ֣י הֹרֵ֔ג אֶת־בִּנְךָ֖ בְּכֹרֶֽךָ׃
And I have said unto thee: Let My son go, that he may serve Me; and thou hast refused to let him go. ‘Behold, I will slay thy first-born.’
Read kabbalistic (mystical) interpretation of Exodus 4:23: and classical mystical sources.