Citing an indirect source occurs when quoting from a source that is quoting from another source. For instance, if you are quoting from a New York Times article that references a story originally published in The Washington Post, you are citing an indirect source. The MLA Handbook recommends taking material from the original source whenever possible. If you need to use indirect quotations, use “qtd. in” to indicate the source consulted. If it is clear in the text that the source is secondhand, “qtd. in” is not needed within the citation.
Ejemplo:
Jones afirmaba que los corredores que "bebían regularmente solían dejar de correr al cabo de unos meses" (qtd. en Salazar 212).