Exodus 4:20-24

So Moses took his wife and his sons and mounted them on a donkey, and returned to the land of Egypt. Moses also took the staff of God in his hand.

And the Lord said to Moses, “When you go back to Egypt, see that you perform before Pharaoh all the wonders which I have put in your power; but I will harden his heart so that he will not let the people go.

Then you shall say to Pharaoh, ‘This is what the Lord says: “Israel is My son, My firstborn.

So I said to you, ‘Let My son go so that he may serve Me’; but you have refused to let him go. Behold, I am going to kill your son, your firstborn.” ’ ”

But it came about at the overnight encampment on the way, that the Lord met Moses, and sought to put him to death.