Pharaoh's Hardened Heart and Redeeming Every Firstborn Son

"And it came to pass when Pharaoh stubbornly refused to let us go" (Exodus 13:15). From where do you say that when the Egyptians grew soft, Pharaoh grew hard? Scripture says, "And I will harden Pharaoh's heart" (Exodus 7:3). When Pharaoh grew soft, the Egyptians grew hard, as it says, "And I will harden the heart of Egypt." When both these and those grew soft, the Omnipresent strengthened their hearts, as it says, "For I have hardened his heart and the heart of his servants" (Exodus 10:1). "Therefore I sacrifice to the LORD all that opens the womb, the males, and every firstborn of my sons I redeem": what does this teach? From where do you say that if a man marries five virgin wives and they bear him five sons, he is obligated to redeem them all? Scripture says, "And every firstborn of my sons I redeem."