Exodus 7:9-10

“When Pharaoh speaks to you saying, ‘Prove yourselves with a miracle,’ then you are to say to Aaron, ‘Take your staff and cast it down before Pharaoh, so that it may become a serpent.’” So Moses and Aaron went in to Pharaoh and did as ADONAI had commanded. Aaron threw down his staff before Pharaoh and before his servants, and it became a serpent.