I have a list of works cited in my bibliography, but one of the sentences I cited within my paper doesn’t have the source I used. It has quotation marks, but I think I forgot to include what source/page it was from.

I already sent my library books back through our interloan library network, so even if I request them all again, it’ll take over a week for all of them to get to me.

Should I guess which one? If I cite the wrong one is that just as bad as plagiarism?