And the Lord spake unto Moses, Go unto Pharaoh, and say unto him, Thus saith the Lord, Let My people go that they may serve Me. This command became a formula in the course of the plagues and was intended to impress Pharaoh by its very repetition.
Related Commentaries of Exodus 8:1
Exodus 8:1
1 And the LORD spake unto Moses, Go unto Pharaoh, and say unto him, Thus saith the LORD, Let my people go, that they may serve me.