Question: Must I cite the source of Divrei Torah?

Answer: The Mishna (Avos 6:6) emphasizes the significance of citing sources: "Whoever says something in the name of its sayer brings redemption to the world." Conversely, the Midrash (Tanchuma Bamidbar #22) writes that one who doesn't cite is included in the verse, "Do not steal from a poor person." (Mishlei 22:22)

The Machaneh Chaim (V:2, C.M. 49) rules that a sofer who publicized the rulings of other rabbonim in his own name is "a thief," for stealing the Torah knowledge of the originator. [This indicates that he recognizes ownership of intellectual property.] Others disagree that theft applies, but include this in geneivas daas or midevar sheker tirchak.

The Gemara (Nazir 56b) indicates that you should mention both the originator of the idea and the immediate source who told you, but do not need to mention the intermediate chain of transmission. However, the Raavad writes (based on Avoda Zara 16b) that the practice is to cite only the original source. Certainly, if you look up the source in the original sefer, there is no need to mention the secondary source that referred you there.

(See Emek Hamishpat, Zechuyos Yotzrim, intro. 31:1-7; ch 2:5, 39:2-4)