One Piece Chapter 6, page 7: Chapter 6: Number One

With a powerful "WHIP!!!!" sound effect, Luffy unleashes his Gum-Gum Whip attack. He stretches his leg out to an incredible length, sweeping it in a wide arc across the courtyard. Zoro ducks down just in time as Luffy's leg sails over him, striking the charging Marines and knocking them off their feet.