Exodus 7:8-10

Then the LORD said to Moses and Aaron, “Pharaoh will demand, ‘Show me a miracle.’ When he does this, say to Aaron, ‘Take your staff and throw it down in front of Pharaoh, and it will become a serpent.’” So Moses and Aaron went to Pharaoh and did what the LORD had commanded them. Aaron threw down his staff before Pharaoh and his officials, and it became a serpent!