Is it plagiarism if I don't mention where I found a quotation originally? Say author A writes on Topic B and in this context uses quotation C which is not from the same field and appears highly original in Topic B. I then read this article, and I am myself writing on Topic B. I find the quote so useful or relevant, that I quote it myself. 

Am I obliged to mention where I found the quote (namely in author A)? Or can I just use the quote? Where are the boundaries here, what do you think is acceptable, what is not? Also, if it is not ethical, that's one thing - but is it also plagiarism?

PS: If anyone has a good title for this sort of question, please feel free to edit. Not sure how to name this.
Yes, this might be very close to plagiarism, but without knowing the details this is hard to tell. The litmus test is this: consider you are author A and somebody else does that: what would you feel?

To be on the save side, just give credit to author A for introducing the quotation C in the context of topic B (but not more). There is no harm done in this way.