Ex 4:21-23
21 GOD said to Moses, "When you get back to Egypt, be prepared: All the wonders that I will do through you, you'll do before Pharaoh. But I will make him stubborn so that he will refuse to let the people go. 22 Then you are to tell Pharaoh, 'GOD's Message: Israel is my son, my firstborn! 23 I told you, "Free my son so that he can serve me." But you refused to free him. So now I'm going to kill your son, your firstborn.'"

This was way in advance!