While your question was already anwered by Bill Barth, I want to try and answer the question that you should have asked:
How to find the right paper to cite for well established facts / problems / theorems / etc.
If this is what you actually intended, than the first paper ever published on the topic is only one possibility. Often a better option is to look for a good review paper on the topic and cite that. A reader is much more likely to gain knowledge from a good review than a (probably decades old) first publication.
Have a look around, how others cite this specific subject and immitate them if you want to make sure not to violate unwritten etiquette. There are basically three possibilities
- No citation: The subject is assumed common knowledge and can probably be found in any standard textbook.
- citing the original paper: this is what you intended. Even if the first publication on the subject will likely not include all of the knowledge on the subject that you have and the reader might thus need to look at other publications as well, this is often done to acknowledge the work that the original author put into this. Make sure that you reference any further work that is needed to understand your work, e.g. during your own summary of the subject.
- citing a recent paper or review article: a) The subject is likely already well established and almost assumed common knowledge. The citation helps the reader to either catch up on some recent developments (paper) or to get a general overview (review). b) The subject has a long and active history. The subject in the formulation that you use probably has no clear first author. Due to numerous modifications / the natural evolution of notation etc. the original publication on the topic is probably of no use to the reader. In this case either a paper with a good introduction or a review can be cited.
Of course combinations of the above are possible as well. E.g. citing the original paper to acknowledge the first author as well as a review article such that the reader might catch up on any results that are already available prior to your work.
Unless you are currently writing a review though, you are not required to dig through generations of publications to find the first paper on a subject. If nobody in your field cites it, you can safely stick with one of the other two options.