Orius
Unrepentant DM Supremacist
Several places. Halfling was what the Hobbits were known as to the men of Gondor primarily. If it wasn't used in the prologue, then it first probably got used by Boromir, who tends to refer to the hobbits as halflings. I believe Faramir and Denethor use the term too, and Pippin hears it quite a bit during his stay in Minas Tirith.Pretty sure the term is actually used in LotR. Can’t remember by who though. Saruman? Eomer? Dunno!
In the Fellowship film, Boromir uses the term halfling in his Ring-fueled rant on Amon Hen; much of those words were taken right from the book.