One Piece Chapter 5, page 4: CHAPTER 5: THE KING OF THE PIRATES AND THE MASTER SWORDSMAN

Koby explains to Zoro that Helmeppo never intended to keep his promise, which is why Luffy punched him. Zoro is stunned, uttering, "WHAT!?" Koby pleads with him, "THE NAVY IS YOUR ENEMY NOW!! PLEASE! IF I UNTIE YOU, WILL YOU HELP LUFFY!?" Zoro remains silent as Koby continues, "HE SAVED MY LIFE! I WON'T ASK YOU TO BECOME A PIRATE, BUT... LUFFY'S REALLY STRONG AND SO ARE YOU! IF YOU TEAM UP......YOU CAN ESCAPE FROM HERE! PLEASE, HELP HIM!"