THE MESSIAH IS A FIRSTBORN

Midrash Rabbah - Exodus 19:7

SANCTIFY UNTO ME ALL THE FIRSTBORN (Exodus 13:1).

R. Nathan said: The Holy One, blessed be He, told Moses: ' Just as I have made Jacob a firstborn, for it says: Israel is My son, My firstborn (Exodus 4:22) so will I make the King Messiah a firstborn, as it says: I also will appoint him firstborn (Psalm 89:28).