The comments on this question led to an interesting conversation about the Dwarvish language (apparently known as Khuzdul among the Dwarves themselves). The conversation centered on Gimli's battle cries at Helm's Deep, as described in the book The Two Towers.
His first battlecry is the longest:
Baruk Khazâd! Khazâd ai-mênu! ["Axes of the Dwarves! The Dwarves are upon you!"]
The second is the shortest:
Khazâd! Khazâd! ["Dwarves! Dwarves!"]
And the third repeats part of the first:
Khazâd ai-mênu! ["The Dwarves are upon you!"]
I asked a Stack Exchange member - who is clearly very knowledgeable about the languages of Middle-earth - whether Gimli was technically misspeaking, since "Khazad" is plural, not singular (i.e., it means "[the] Dwarves", not "[a] Dwarf"), and Gimli is the only Dwarf present (so he technically should have said "Axe of the/a Dwarf! A Dwarf is upon you!"). This isn't really a problem, because Gimli was presumably using the standard Dwarf battlecry, not trying to accurately describe the number of Dwarves in the area.
However, this Stack Exchange member who knows a great deal about the languages of Middle-earth said two things that intrigued me:
1. These battle cries may be the only examples of Dwarvish (Khuzdul) that we have from Tolkien's own pen
2. Tolkien "may not have intended for there to be a singular/plural distinction in Dwarvish at all, though of course David Salo certainly made one for neo-Khuzdul."
The first suggestion I find very interesting, and I wonder if it is true; the second I find shocking - Tolkien was, after all, a philologist (i.e., a linguist), and I would have thought (especially considering Tolkien's attention to detail, particularly in regards to languages) that he would address plural/singular distinctions as a matter of course.
Tolkien Gateway's entry on Khazâd/Khuzd addresses the plural/singular issue in relation to the Dwarvish (Khuzdul) words for Dwarf (Khuzd) and Dwarves (Khazâd), but I don't know if it is based on Tolkien's own writing. Here is what it has to say:
Khuzd pl. Khazâd was the Khuzdul word for the "Dwarves".
The word comes from the Root Kh-Z-D; it is also visible in the words Khuzdul and perhaps Nulukkhizdîn.
The plural form Khazâd is the basis of Quenya casar and Sindarin hadhod. The Adûnaic word hazad, meaning "seven", is presumably also related.
In earlier versions, the plural was Khuzûd
Another entry (on "Racism in Tolkien's Works") includes the following quote from Tolkien himself:
The dwarves of course are quite obviously - wouldn't you say that in many ways they remind you of the Jews? Their words are Semitic obviously, constructed to be Semitic.
As far as I know (from studying textual criticism of the bible, which has exposed me to a little Aramaic and Hebrew, both of which are Semitic languages), there are indeed plural and singular word forms in Semitic languages. Granted, Tolkien said that "Their words are... constructed to be Semitic", not that the language mirrors all aspects of Semitic languages, but I have a hard time believing that a philologist would adopt Semitic words but dispose of the languages' pre-existing singular/plural distinctions.
So I have two questions:
1. Are the phrases I mentioned above the only examples of Dwarvish (Khuzdul) speech/writing that appear in Tolkien's work? If not, what are the other examples?
2. Do we have any evidence that Tolkien himself intended for Dwarvish (Khuzdul) to include singular/plural distinctions?
Note 1: At first I was tempted to say that the doors of Moria are engraved with Dwarvish words, but after further reflection, I think it was actually Elvish writing. If I am wrong, and it was actually Dwarvish, please correct me in your answer - consider this a sub-question.
Note 2: To give credit where credit is due, the very knowledgeable SE member who I mentioned above, and who inspired this question, was Janus Bahs Jacquet.
Note 3: A resource mentioned and linked in the first answer submitted for this question seems to support the claim that, aside from a few names, Gimli's battle cries are indeed the only Dwarvish phrases we have from Tolkien himself:
As has already been mentioned, our Khuzdul corpus is very small. There are a few names, like Khazad-dûm and Zirak-zigil, the inscription on Balin's tomb, and a battle cry: Baruk Khazâd! Khazâd ai-mênu! "Axes of the Dwarves! The Dwarves are upon you!"