How to efficiently find citations for quotations after writing? I wrote a long document about mathematics with a lot of quotations.
I must provide a citation for every quotation, but did not do this so far.
Now I’m stuck because I can’t efficiently find the origins of these quotations, given the plethora of available references.

What kind of workflow can I use to handle this task efficiently?
Presumably you have an idea of which references you made the most use of, and can skim through them again. If you really have no idea, the most efficient way to find the source I can think of is Google (or your search engine of preference). Simply copy and paste (a text part of) a quotation, put quotation marks around it, and search for it. Then correlate the findings with your set of sources. Alternatively, if you have say PDFs of all/most of your sources, you can use advanced search features to search for a quotation in multiple files at the same time. Sometimes PDFs are scanned and non-searchable, so googling can be more capable.

Now, since you mention mathematics, let me also say that these techniques obviously don't work as well for equations. For those you can try searching for related terms/names, but it can be difficult to find the exact source that way. It's probably more efficient to skim through your sources for this, which works well except for in-text equations. The latter tend to require careful reading.

Overall though, consider this a lesson to cite as you write. Do this even for notes only you will see - if it's a useful source, you'll want to remember it later!