One Piece Chapter 5.3, page 15-19: The Pirate King and the Great Swordsman

The next day, Zoro learns that Kuina died from falling down the stairs. Devastated, Zoro begs Koushirou to give him Kuina's sword, the Wado Ichimonji. Holding the blade, Zoro tearfully vows to become so famous that his name will reach the heavens.

Back in the present, Zoro remembers his promise as the Marines prepare to fire. Suddenly, Luffy launches himself from the Marine tower using "GOMU-GOMU... ROCKET!!" just as Morgan orders his men to fire.

Luffy lands directly in front of Zoro and Coby, taking the full force of the firing squad's bullets. To everyone's utter shock, the bullets stretch his rubber body instead of piercing him.

Luffy laughs as his rubber body snaps back, bouncing the bullets directly back at the terrified Marines and scattering them.

Luffy proudly declares his dream to become the Pirate King. He presents Zoro with all three of his katanas, offering them back if Zoro joins his crew. Facing execution otherwise, Zoro smiles and agrees to become a pirate.