Exodus 4:19-23

And the LORD said unto Moses in Midian, Go, return into Egypt: for all the men are dead which sought thy life. And Moses took his wife and his sons, and set them upon an ass, and he returned to the land of Egypt: and Moses took the rod of God in his hand. 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: but I will harden his heart, that he shall not let the people go. And thou shalt say unto Pharaoh, Thus saith the LORD, Israel is my son, even my firstborn: and I say unto thee, Let my son go, that he may serve me: and if thou refuse to let him go, behold, I will slay thy son, even thy firstborn.