One Piece Chapter 30, page 3: GREAT!!!

Down at the coastal slope, the scene is chaotic. Dozens of Black Cat Pirates lie defeated and groaning on the ground. Jango stands in utter disbelief in the middle of his fallen crew. At the top of the slope, Luffy and Zoro stand triumphantly. Usopp, sitting on the ground nearby, is amazed by their strength and says, "I DIDN'T KNOW YOU GUYS WERE THIS STRONG..." to which Luffy confidently replies, "OF COURSE!"