One Piece Chapter 35, page 7: THE STEEP SLOPE AGAIN

Kuro deduces that Luffy possesses a Devil Fruit power. Luffy cheerfully confirms, "YUP! I ATE A 'GOMU GOMU FRUIT'!! AND I'VE BECOME A RUBBERMAN!!" The Black Cat Pirates are shocked to learn that Devil Fruits actually exist, and Jango realizes Luffy's stretching arm wasn't an illusion.