One Piece Chapter 16, page 5: VERSUS BUGGY'S CREW!

Mohji urgently warns Buggy that Luffy is a rubber man who ate a Devil Fruit, explaining why the Buggy Ball bounced off him. Buggy is shocked to hear this and angrily demands to know why Mohji didn't tell him sooner, while Luffy playfully demonstrates his stretchy cheeks to Nami.