Exodus 7:1-9

So the LORD said to Moses: “See, I have made you as God to Pharaoh, and Aaron your brother shall be your prophet. You shall speak all that I command you. And Aaron your brother shall tell Pharaoh to send the children of Israel out of his land. And I will harden Pharaoh’s heart, and multiply My signs and My wonders in the land of Egypt. But Pharaoh will not heed you, so that I may lay My hand on Egypt and bring My armies and My people, the children of Israel, out of the land of Egypt by great judgments. And the Egyptians shall know that I am the LORD, when I stretch out My hand on Egypt and bring out the children of Israel from among them.” Then Moses and Aaron did so; just as the LORD commanded them, so they did. And Moses was eighty years old and Aaron eighty-three years old when they spoke to Pharaoh. Then the LORD spoke to Moses and Aaron, saying, “When Pharaoh speaks to you, saying, ‘Show a miracle for yourselves,’ then you shall say to Aaron, ‘Take your rod and cast it before Pharaoh, and let it become a serpent.’ ”