American Standard Version (ASV):
and I have said unto thee, Let my son go, that he may serve me; and thou hast refused to let him go: behold, I will slay thy son, thy first-born.
King James Version (KJV):
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.