And the LORD said to Moses, "When you go back to Egypt, see that you do all those wonders before Pharaoh which I have put in your hand. But I will harden his heart, so that he will not let the people go.
Exodus 4:21
Surrounding context
4:20 Then Moses took his wife and his sons and set them on a donkey, and he returned to the land of Egypt. And Moses took the rod of God in his hand.
4:21 And the LORD said to Moses, "When you go back to Egypt, see that you do all those wonders before Pharaoh which I have put in your hand. But I will harden his heart, so that he will not let the people go.
4:22 Then you shall say to Pharaoh, "Thus says the LORD: "Israel is My son, My firstborn.