Exodus 7:9-10

“When Pharaoh says to you, ‘Give me a sign to prove that God has sent you,’ tell Aaron, ‘Take your shepherd’s staff and throw it down in front of Pharaoh,’ and it will become a large snake.” Moses and Aaron went to Pharaoh and did as the LORD had commanded. Aaron threw his staff down in front of Pharaoh and his officials, and it became a large snake.