One Piece Chapter 16, page 15-16: VERSUS BUGGY'S CREW!

To everyone's absolute horror, Zoro deliberately stabs his own wound deeper, causing blood to spray. Cabaji, Nami, Buggy, and Luffy stare in shock as Zoro intentionally handicaps himself.

Zoro declares his goal is to be the world's greatest swordsman, asking Cabaji if this self-inflicted wound is enough of a handicap. Luffy grins in admiration, while a sweating Cabaji realizes Zoro is completely serious about defeating him.