One Piece Chapter 4, page 5-6: Chapter 4: The Great Captain Morgan

Zoro explains that he has his own mission and refuses to become a "stinking pirate." Luffy questions if he thinks he's too good to be a pirate, pointing out that everyone already thinks Zoro is a demon bounty hunter. Zoro firmly states, "I live by my own code... I've never done anything I regret, and I don't intend to in the future. I don't care what people think. Which is why I'll never be a pirate!!" Luffy mentions he heard Zoro can use a sword, and Zoro boasts, "Hmph! If I wasn't tied up, I'd show you..." Luffy stubbornly declares, "Sorry, but I've made up my mind! You're gonna join my crew!!" Zoro screams back, "I don't care what you've decided!!

Luffy asks Zoro where his swords are. Zoro explains that Captain Morgan's idiot son took them, noting, "Next to my life, those swords are my dearest treasures." Luffy gets an idea and offers to retrieve them from Helmeppo. However, he adds a condition: "Then, if you want your swords back... you'll have to join my crew!" Zoro angrily calls him a "little rat," but Luffy just laughs and runs off toward the Marine fortress, shouting, "See ya soon!" Zoro watches him go, muttering, "He's going into the fortress... That's one dumb pirate!