There is no proper way of citing LaTeX.
LaTeX is not created by a well-known bunch of people or by a institution you can refer to. LaTeX is not a book (there is a proper way of citing books, that is to say with authors, title, publisher, date, ...). LaTeX is composed by many packages and macros (etc.) with really many authors. So you cannot say "thanks to LaTeX software created by X with the support of Y" (because it will incomplete and therefore not correct).
However, you can choose to mention the historical author of LaTeX, Leslie Lamport and Donald Knuth (for TeX and the original idea). Or you can cite some very famous books about LaTeX like the The LaTeX Companion (but both solutions are not citing directly LaTeX but rather LaTeX history or a LaTeX handbook.
In my opinion, the best way to refer to LaTeX is to talk about the LaTeX community (which is a abstraction) and insert a link to the LaTeX project website.
For example : "I thank the LaTex community for this very useful document preparation system and document markup language".