Crantag is right. You should strike any quote you cannot confirm. But I would go further.
Concerning Drlee's answer, I don't think that this phrase is a quip. In any case, the writer who attributes this quote clearly does not consider it a quip.
(First I readily admit that I spend practically no time editing my posts here for grammar. It takes the fun out of posting for me. That said.)
You are a book editor. Having been 'edited' many times I believe my editors to be of the opinion that they were not there only as fact-checkers but also to make my piece as effective and readable as possible. For example, you are checking grammar, are you not? No split infinitives or errors of voice will get past you, will they?
We have not found a source for this so-called quote. You are responsible for that, not him. IF Marx said anything like this (and he probably didn't...witness his established dislike of anarchists) then this quote is, at best, a
paring down of a much more developed idea. So much so that an editor would be careful lest the author is taking something out of context entirely. The only way to know is to have the entire quote in front of you.
Sometimes an author wants to make a point but does not want to attribute it to him/herself. The "it has been said" device can acceptably be used to do it. In this case, unless the author is specifically discussing Marx and his relationship with anarchists it seems the only reason to use this "quote" is to borrow some of Marx' credibility or innate gravitas. So of the quote was changed to "it has been said, there can be no revolution without anarchists......." what is lost?
Looking back I note that you said ,
"I also forgot to mention that the writer says that Marx said this phrase to some socialists who had negative attitude against Intern's anarchists. Excuse me!" This seems to me to be the key. Here the author seems to be using the quote as evidence of a particular attitude Marx held in a specific situation. As an editor you can't allow an evidential quote to stand unproved. To do so is to allow your charge to place his entire thesis in jeopardy. No editor should do that.
So whether this is a quip or a strong piece of evidence you, the editor, cannot allow it to stand. You have to strike it or call upon the author to replace it with something verifiable.
That is what editors do.