Let's say you're writing a paper about a novel problem in Mathematics. While writing the paper you remember an interesting result concerning a different related problem. You can derive that result from memory, but you cannot remember where exactly you've read it - it could've been a paper, a pre-print on Arxiv.org, a post on Math.SE, a book from the library, a lecture that you've attended, etc. No matter how hard you try you cannot find the exact source.
What is one supposed to do in this situation?