Israel is my son, even my firstborn:

22 And thou shalt say unto Pharaoh, Thus saith the Lord, Israel is my son, even my firstborn:?
23 And I say unto thee, Let my son go, that he may serve me: and if thou refuse to let him go, behold, I will slay thy son, even thy firstborn. ?

The dedication of the first-born had its origin in the earliest times. God had promised to give the First-born of heaven to save the sinner. This gift was to be acknowledged in every household by the consecration of the first-born son. He was to be devoted to the priesthood, as a representative of Christ among men. {DA 51.1}
In the deliverance of Israel from Egypt, the dedication of the first-born was again commanded. While the children of Israel were in bondage to the Egyptians, the Lord directed Moses to go to Pharaoh, king of Egypt, and say, "Thus saith the Lord, Israel is My son, even My first-born: and I say unto thee, Let My son go, that he may serve Me: and if thou refuse to let him go, behold, I will slay thy son, even thy first-born." Exodus 4:22, 23. {DA 51.2}

If Pharaoh had agreed to let the Israelites go, the people of Egypt would have been spared a lot of sorrow