I have a question about references. I was reading a paper and I emailed the author asked him about something if it is already known or not. Let make it as example, I asked about basic property say $P$ and he stated the property $P$ in general case he said that he did not know a reference for this property. What's more, he did not believe if anyone write the property in this general case. My question, in case I need to use his idea in my project, what should I do ? since there is no reference for that just his email.