Exodus 7:9-10

When Pharaoh shall speak to you, saying, Do a miracle for yourselves, — then thou shalt say unto Aaron, Take thy staff and cast it before Pharaoh — it will become a serpent. And Moses and Aaron went in to Pharaoh, and did so, as Jehovah had commanded; and Aaron cast down his staff before Pharaoh, and before his bondmen, and it became a serpent.