Exodus 4:21
20 Then Moses took his wife and sons and put them on a donkey and headed back to the land of Egypt, and Moses took the staff of God in his hand. 21 The Lord said to Moses, “When you go back to Egypt, see that you do before Pharaoh all the wonders I have put under your control. But I will harden his heart and he will not let the people go. 22 You must say to Pharaoh, ‘This is what the Lord has said, “Israel is my son, my firstborn,
Exodus 7:13
11 Then Pharaoh also summoned wise men and sorcerers, and the magicians of Egypt by their secret arts did the same thing. 12 Each man threw down his staff, and the staffs became snakes. But Aaron’s staff swallowed up their staffs. 13 Yet Pharaoh’s heart became hard, and he did not listen to them, just as the Lord had predicted. 14 The Lord said to Moses, “Pharaoh’s heart is hard; he refuses to release the people.
Exodus 9:35
33 So Moses left Pharaoh, went out of the city, and spread out his hands to the Lord, and the thunder and the hail ceased, and the rain stopped pouring on the earth. 34 When Pharaoh saw that the rain and hail and thunder ceased, he sinned again: both he and his servants hardened their hearts. 35 So Pharaoh’s heart remained hard, and he did not release the Israelites, as the Lord had predicted through Moses.
Notes and References
"... Exodus verse 21b is the first occurrence of a formula used by the Redactor to organize the E and P accounts of the plagues into a united narrative, thus: The E accounts of the plagues of the insect swarm and the livestock epidemic conclude, “And Pharaoh’s heart was heavy, and he did not let the people go” (8:28; 9:7). The P accounts of the plagues of lice and boils and also the P account of the staffs becoming serpents conclude, “And Pharaoh's heart was strong, and he did not listen to them as YHWH had spoken” (7:13; 8:15; 9:12). The plague of blood is both E and P, and it concludes with the P formulation: “And Pharaoh's heart was heavy, and he did not let the people go.” The plague of frogs is also combined E and P, and it ends in 8:11 both with part of the E conclusion (“he made his heart heavy”) and with part of the P conclusion (“he did not listen to them as YHWH had spoken”). It is not surprising that P accounts have the P conclusion, E accounts have the E conclusion, and combined accounts have either a P or a combined conclusion. But then the E plague of hail has what has been the P conclusion, “And Pharaoh's heart was strong, and he did not let the children of Israel go as YHWH had spoken” (9:35). Then the E plagues of locusts and darkness also conclude with a P formula (10:20, 27), and the final meeting between Moses and Pharaoh that ensues is likewise an E text followed by a P conclusion. lt appears that the Redactor has combined the P and the E accounts of the plagues and has united them by drawing on the P formula and distributing it through the combined version. This is confirmed by the fact that the formula also appears here in 4:21b. It is awkward in this context, and again it is a formula derived from P in the middle of an E text ..."
Friedman, Richard Elliott The Bible with Sources Revealed: A New View Into the Five Books of Moses (p. 125) Harper San Francisco, 2005