Khuzdul

Khuzdul is the own tongue of the Dwarves, invented for them by their creator Aulë. Like the other languages it changed with time, but so slowly that even in the Third Age, speakers of all its different varieties understood each other. Although Khuzdul was not truly speaking a secret language, Dwarves were reluctant to share it with other peoples; they seem to have regarded it as something of their own. Some Dwarves like Thorin's kin adopted other languages in daily use, but they went on using Khuzdul among themselves.

Inscription on the tomb of Balin in Moria - The Lord of the Rings, book II, chapter 4.

Dwarvish battle cry - The Lord of the Rings, book III, chapter 7.

Khuzdul names - picked up in The Lord of the Rings, The Silmarillion, Unfinished Tales and the collection The History of Middle-earth.