Exodus 4:21

And the Lord said unto Moses, When thou goest to return into Egypt, see that thou do all those wonders before Pharaoh which I have put in thine hand. The first commission was here repeated and explained, in order to impress every detail upon Moses' mind. After his return to Egypt he was to perform all the wonders, all the terrible signs, which the Lord had placed in his hand to do. There would be need of great firmness and courage in dealing with Pharaoh. But I will harden his heart that he shall not let the people go. In His omniscience the Lord here anticipates. He knew that Pharaoh would harden his heart willfully and maliciously, would refuse to heed the successive appeals that would be made, and therefore God announces the final judgment upon the Egyptian king, the condemnation which would make it impossible for him to be converted in the end.