Exodus 10:1, 3

And the LORD said unto Moses, Go in unto Pharaoh: for I have hardened his heart, and the heart of his servants, that I might shew these my signs before him

And Moses and Aaron came in unto Pharaoh, and said unto him, Thus saith the LORD God of the Hebrews, How long wilt thou refuse to humble thyself before me? let my people go, that they may serve me.