One Piece Chapter 7, page 12: Chapter 7: Friends

Luffy rubs his cheek, smiling slightly, while Ripper, the Marines, Rika, and Ririka look on in shock. Zoro smiles, realizing what Luffy is doing. Koby stands panting, "HUF HUF", sweating and terrified of what he just did. Luffy suddenly gets up, angry, and yells, "WHY, YOU--!!" as he punches Koby hard in the face with a loud "WHAM!!" sound effect.