One Piece Chapter 2.3, page 20: That Boy "The Straw Hat Wearing Luffy"

Luffy stretches his arm far back, preparing his attack. Coby, Alvida, and the pirates watch in utter disbelief, shouting, "NO WAAAY!!!" and "HUH?!" Luffy yells, "GOMU GOMU NO... PISTOL..." and unleashes a powerful, stretched punch that strikes Alvida squarely in the face. The force of the blow sends her flying backward through the air, knocking her hat off.