And thou shalt say unto Pharaoh, Thus saith the LORD, Israel is my son, even my {k} firstborn:

(k) Meaning, most dear to him.

Wesley's Notes

4:22 Thus saith the Lord - This is the first time that preface is used by any man, which afterwards is used so frequently by all the prophets: Israel is my son, my first - born - Precious in my sight, honourable, and dear to me.

Matthew Henry's Concise Commentary

4:18-23 After God had appeared in the bush, he often spake to Moses. Pharaoh had hardened his own heart against the groans and cries of the oppressed Israelites; and now God, in the way of righteous judgment, hardens his heart against the teaching of the miracles, and the terror of the plagues. But whether Pharaoh will hear, or whether he will forbear, Moses must tell him, Thus saith the Lord. He must demand a discharge for Israel, Let my son go; not only my servant, whom thou hast no right to detain, but my son. It is my son that serves me, and therefore must be spared, must be pleaded for. In case of refusal I will slay thy son, even thy first-born. As men deal with God's people, let them expect so to be dealt with.