One Piece Chapter 39, page 18: FOR WHOM THE BELL TOLLS

On the slope, Luffy snaps his head forward with incredible force, completing his attack: "...BELL!" (Gomu Gomu no Kane). His forehead slams directly into Kuro's face with a devastating impact (SFX: DO-GAAAN!!!). Kuro's glasses shatter instantly as he takes the full force of the headbutt, his eyes wide with shock and pain ("!!!?").