True.
If the expert that wrote the source is truly an expert and had studied in the field (along with earning some kind of degree in it), then the source is more "credible" then say, a source that was a random person's opinion.
Fail to cite sources properly.
The Point of having "plagiarism" is using information that is not yours, and not giving credibility to the one who made it. In this case, saying a quote without citing the author of it, or writing a quote without citing the author is the same thing.
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