Exodus 4:23
and I have said to you, “Let my son go, that he may serve me;” and you have refused to let him go. Behold, I will kill your firstborn son.’”
World English Bible (Public Domain)
In context
Before · Exodus 4:21–22
21The LORD said to Moses, “When you go back into Egypt, see that you do before Pharaoh all the wonders which I have put in your hand, but I will harden his heart and he will not let the people go. 22You shall tell Pharaoh, ‘The LORD says, Israel is my son, my firstborn,
23and I have said to you, “Let my son go, that he may serve me;” and you have refused to let him go. Behold, I will kill your firstborn son.’”