One Piece Chapter 3, page 20: Chapter 3: Enter Zolo: Pirate Hunter

Enraged by Helmeppo's cruel deception, Luffy punches Helmeppo squarely in the face, sending him flying. Koby desperately tries to hold Luffy back, warning him that attacking Helmeppo means making an enemy of the Navy. But Luffy, completely determined, declares that he has made his decision: he is going to ask Zolo to join his crew.