MLA recommends citing the original source wherever possible, rather than the source in which it is quoted or reproduced.
If this isn’t possible, cite the secondary source and use “qtd. in” (quoted in) in your MLA in-text citation. For example: (qtd. in Smith 233)
If a source is reproduced in full within another source (e.g. an image within a PowerPoint or a poem in an article), give details of the original source first, then include details of the secondary source as a container. For example:
Picasso, Pablo. Guernica. 1937. Introduction to Art History, taught by James Wilson, 24 Sep. 2019, Duke University, Durham. Slide 7.