One Piece Chapter 43, page 8: Sanji Enters

The Marine cannon fires with a loud blast. Usopp screams, "HE FIRED!" and panics, "LUFFY, WHAT'RE YOU DOING!?" Luffy leaps into action, shouting, "LEAVE IT TO ME! GOMU GOMU NO... BALLOON!!!" He inhales deeply, inflating his body into a giant, round rubber ball. The cannonball strikes Luffy's inflated stomach, stretching his rubber skin but failing to pierce it. Luffy grins and yells, "YOU CAN HAVE YOUR CANNONBALL BACK!" as his body prepares to slingshot the projectile back. Fullbody and his crew watch in utter disbelief, crying out, "WHA!?" and "THE HELL!?"