Shemot 7:9-10

When Pharaoh shall speak unto you, saying, Show a mofet (miracle) for you: then thou shalt say unto Aharon, Take thy matteh, and cast it before Pharaoh, and it shall become a serpent. And Moshe and Aharon went in unto Pharaoh, and they did so as HASHEM had commanded: and Aharon cast down his matteh before Pharaoh, and his avadim, and it became a serpent.