How do I cite quotations from one source found in another source?
Purdue OWL provides these instructions for citing indirect quotations:
Citing Indirect Sources
Sometimes you may have to use an indirect source. An indirect source is a source cited in another source. For such indirect quotations, use "qtd. in" to indicate the source you actually consulted. For example:
Ravitch argues that high schools are pressured to act as "social service centers, and they don't do that well" (qtd. in Weisman 259).
Note that, in most cases, a responsible researcher will attempt to find the original source, rather than citing an indirect source.
These same rules are also stated in the MLA Handbook, in section 6.4.7. Here is an example it provides:
Samuel Johnson admitted that Edmund Burke was an "extraordinary man" (qtd. in Boswell 2: 450)
Then on the works cited page:
Boswell, James. The Life of Johnson. Ed. George Birkbeck Hill and L.F. Powell. 6 vols. Oxford: Clarendon, 1934-50. Print.