One Piece Chapter 2, page 17: THEY CALL HIM "STRAW HAT LUFFY"

Alvida swings her spiked iron mace with full force, screaming, "YOU'RE BOTH GOING TO DIE!!" Koby cries out, "L-LUFFY...!!" but Luffy steps in front of him, smiling, "GOOD FOR YOU, KOBY!! IT MAKES NO DIFFERENCE TO ME..." The mace strikes Luffy directly on the head with a heavy "WHAM!!" impact. However, Luffy remains completely unharmed, smiling widely as he says, "THAT DIDN'T HURT! 'CAUSE I'M MADE OF RUBBER!" Alvida is shocked, stammering, "IMPOSSIBLE! NO ONE SURVIVES THE IRON MACE!!"