One Piece Chapter 22, page 8: YOU ARE A RARE AND PRECIOUS ANIMAL TOO

A loud gunshot rings out, and a bullet strikes Luffy directly in the chest. Nami screams in terror, but Luffy's rubber body simply stretches to absorb the impact before snapping back and sending the bullet flying harmlessly away. Nami is amazed to see that bullets do not hurt him. The hidden shooter gasps in shock, asking what kind of creature Luffy is, to which Luffy retorts that that is their line.