Albert Barnes Commentary Exodus 4:22

"And thou shalt say unto Pharaoh, Thus saith Jehovah, Israel is my son, my first-born:" — Exodus 4:22 (ASV)

My firstborn - The expression would be perfectly intelligible to Pharaoh, whose official designation was “son of Ra.” In numberless inscriptions, the Pharaohs are styled “own sons” or “beloved sons” of the deity. It is here applied for the first time to Israel; and as we learn from (Exodus 4:23), emphatically in antithesis to Pharaoh’s own firstborn.