From the Skypiea arc, Luffy is in that giant snake, and suddenly he encounters Nami and the rest:
> Luffy: "Ey, weird guy, Nami, what are you doing in this magic cave?"
(...)
> Luffy: "I see, that's really bad, you got eaten by a giant snake..."
> Nami: "You're in the snake's stomach too!!"
(...)
> Luffy: "Ah, it's true! We must find this guy's butthole now!"
> Nami: "BAD IDEA!" *slaps*
> Luffy: "If we've got eaten, we just go out his ass, right?"
EDIT:
Hehe, and though it isn't a quote, i really like this one:
Chapter 276, page 6... > Eneru: *very cold stare*