One Piece Chapter 2.3, page 19: That Boy "The Straw Hat Wearing Luffy"

Luffy quickly grabs Coby and pulls him behind his back, saying, "WELL SAID, COBY! NOW GET BEHIND ME!!" Alvida swings her massive spiked iron mace down with full force, screaming, "BOTH OF YOU... HAVE TO DIE!" The mace strikes Luffy directly on the head with a massive impact. Coby screams, "LU... LUFFY-SAN!!" However, Luffy is completely unharmed. He smiles widely and says, "IT'S USELESS AGAINST ME. BE-CAUSE MY BODY IS RUBBER." Alvida's eyes widen in shock as she stammers, "IM...THAT'S IMPOSSIBLE!!! MY BLUDGEON!" The pirates gasp in disbelief.