exodus 7:4-9
4 When Pharaoh does not listen to you, then I will lay My hand on Egypt and bring out My hosts, My people the sons of Israel, from the land of Egypt by great judgments. 5 The Egyptians shall know that I am the Lord, when I stretch out My hand on Egypt and bring out the sons of Israel from their midst.” 6 So Moses and Aaron did it; as the Lord commanded them, thus they did. 7 Moses was eighty years old and Aaron eighty-three, when they spoke to Pharaoh.
8 Now the Lord spoke to Moses and Aaron, saying, 9 “When Pharaoh speaks to you, saying, ‘Work a miracle,’ then you shall say to Aaron, ‘Take your staff and throw it down before Pharaoh, that it may become a serpent.’