Sometimes the author of a paper will refer to a paper by another author. If the original paper is relevant to your research, then you should make every effort to find it and read it. If this is not possible, then the in-text citation should include both the source that you have read and the source that you have not read, but only the source that you have actually read should be included in the references.
Citing an original work from a secondary source:
a. Secondary citation within the text:
According to Freud (as cited in Skinner, 1923), the characteristics ….
b. The document used is cited in the reference list:
Skinner, B. F. (1974). About behavioralism. New York, NY: Knopf.
Notice that Freud is mentioned in the body of the paper so the reader would understand that any ideas being cited even though the actual book that discussed Freud's ideas was actual written by Skinner.