Thus, although Moses and Aaron performed all these wonders in Pharaoh’s presence, the Lord hardened Pharaoh’s heart, and he would not let the Israelites go from his land.
When Pharaoh stubbornly refused to let us go, the Lord killed every firstborn in the land of Egypt, the firstborn of human being and beast alike. That is why I sacrifice to the Lord every male that opens the womb, and why I ransom every firstborn of my sons.’
The Lord said to Moses: On your return to Egypt, see that you perform before Pharaoh all the wonders I have put in your power. But I will harden his heart and he will not let the people go.
The Lord answered Moses: Now you will see what I will do to Pharaoh. For by a strong hand, he will let them go; by a strong hand, he will drive them from his land. Confirmation of the Promise to the Ancestors.
But although Pharaoh found upon inquiry that not even so much as one of the livestock of the Israelites had died, he remained obstinate and would not let the people go. Sixth Plague: The Boils.