One Piece Chapter 6, page 9: Chapter 6: Number One

The Marines crash heavily to the ground, completely defeated. Captain Morgan watches in utter shock, gasping, "!!?" Zoro, staring at Luffy's leg as it snaps back to normal, asks in amazement, "WHAT THE DEVIL ARE YOU!?" Koby is equally thrilled, cheering, "WOW! HE'S INCREDIBLE!" Luffy smiles proudly, retracting his leg, and happily declares, "I'M A......RUBBER MAN!!!"