This is the whole story of the Israelites freedom from Egypt!
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.'"