One Piece Chapter 3, page 6: Chapter 3: Enter Zolo: Pirate Hunter

Koby tells Luffy, "You won't find him just by peeking over the fence. He's probably deep within the bowels of the prison..." but Luffy spots someone tied up in the middle of the courtyard. Koby climbs up to look and is instantly paralyzed with fear. He recognizes the black bandanna and haramaki sash, confirming that the menacing figure tied to the post is indeed the infamous Roronoa Zolo.