In your research, you will often encounter resources where an author refers to another researcher's work. This source within a source is known as a 'secondary source' or 'indirect source'.
Citing a source from a secondary source is generally discouraged since authors are expected to have examined the works they cite. However, it is not always possible to obtain the original publication - it may be published in another language, or in a book or journal which is out of print.
To cite a source from a secondary source, mention both the original and secondary sources in the text, and list only the secondary source in the works-cited list entry. MLA requires you to use the abbreviation qtd. in ("quoted in") before the indirect source you cite in your parenthetical reference. You may document the original source in a note if needed.
For example, Louis Zukofsky’s article from 1931 is being quoted in Bonnie Costello’s 1981 book on page 78. If Louis Zukofsky’s work was also relevant to your own assignment you might decide that you also need to refer to Zukofsky’s article in your writing. Where possible, you should try to obtain the original published work by Louis Zukofsky - in this case, a 1931 journal article - and read through the article to form your own opinions on his research. However, if it's impossible to access the original article, you may cite the secondary source.
The example above will be cited as below.
In Louis Zukofsky’s “Sincerity and Objectification” (qtd. in Costello 78) ...
Costello, Bonnie. Marianne Moore: Imaginary Possessions. Harvard UP, 1981.