Is it plagiarism if I don't mention where I found a quotation originally?
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.