One Piece Chapter 2, page 19: THEY CALL HIM "STRAW HAT LUFFY"

Alvida lies defeated on the ground. Her crew is terrified, crying, "HE BEAT LADY ALVIDA! HE'S NOT HUMAN! CAPTAIN!!!" Luffy turns to the remaining pirates and commands, "GIVE KOBY A DINGHY! HE'S GONNA JOIN THE NAVY!" The pirates quickly salute, stammering, "Y-YESSIR..." Luffy laughs, "HA HA HA!" while Koby sheds tears of gratitude, whispering, "LUFFY..."