How do we know God hardened Pharaoh's heart?

Throughout the narrative in Exodus, it is stated repeatedly that God hardened Pharaoh's heart. For example, in Exodus 7:14, God tells Moses that Pharaoh's heart is hardened, indicating divine action. Furthermore, Romans 9:17 connects Pharaoh's hardening to God's sovereignty over nations, proving that the hardening served God's purpose of demonstrating His glory through judgment and the eventual deliverance of Israel. This doctrine reflects the Reformed understanding of God's sovereignty over human free will, showcasing that even the stubbornness of Pharaoh was orchestrated by God's sovereign will.