Exodus 4:23

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

εἶπα δέ σοι Ἐξαπόστειλον τὸν λαόν μου ἵνα μοι λατρεύσῃ· εἰ μὲν οὖν μὴ βούλει ἐξαποστεῖλαι αὐτούς, δρᾶ οὖν, ἐγὼ ἀποκτέννω τὸν υἱόν σου τὸν πρωτότοκον.

Greek text from Swete's edition, CC BY-SA 4.0

Ya te he dicho que dejes ir á mi hijo, para que me sirva, mas no has querido dejarlo ir: he aquí yo voy á matar á tu hijo, tu primogénito.

and I told you to let My son go so that he may worship Me. But since you have refused to let him go, behold, I will kill your firstborn son!’”