וְאָמַרְתָּ֖ אֶל־פַּרְעֹ֑ה כֹּ֚ה אָמַ֣ר יְהוָ֔ה בְּנִ֥י בְכֹרִ֖י יִשְׂרָאֵֽל

Then you shall say to Pharaoh, ‘Thus says the LORD: Israel is My first-born son. 

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

I have said to you, “Let My son go, that he may worship Me,” yet you refuse to let him go. Now I will slay your first-born son.’” 

Exodus 4:22-23