Exodus 4:23
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.
And I say unto thee, let my son go, that he may serve me,.... Worship God according to his will in the place he had designed for him, and where he might be safe and free; and which service was due from him as a son, and to be performed not in a servile way, but in a filial manner, and therefore as a servant he could demand his dismission, and much more as his son; and this is required in an authoritative way, for saying is here commanding, insisting on it as a point of right to be done:
and if thou refuse to let him go, behold, I will slay thy son, even thy firstborn; meaning, not only in a strict and literal sense Pharaoh's firstborn son, and heir to his crown, but the firstborn of all his subjects, which in a civil sense were his. This was not to be said to Pharaoh at the first opening of his commission to him, but after all methods had been tried, and the several other plagues designed were inflicted on him to no purpose, he was to be told this, which was the last plague, and succeeded; but this is told to Moses before hand, that when other messages he should be sent with to him, and all that should be done by him would prove ineffectual, this, when sent with and performed, would have the desired effect.