On citing "common knowledge" statements I am an undergraduate student with no experience at all.

If you are writing a research paper, that you are planning to submit for publication, how do you state that a particular statement/fact is commonly known to all and it's dicoverer/inventor is not known, and that there exists no research paper that introduced the concept but there do exist various books/websites that discuss the very concepts and a few derivatives of the concept?

I am not able to cite a particular statement/bunch of statements as I am not able to find any research paper related to that at all.

The particular thing in question is: https://math.stackexchange.com/questions/161661/source-of-probably-the-most-simplest-math-trick

What should I do?
If you found a book that discusses the statement, and you really can't track down where it comes from (does the book cite anything?), then citing the book (with chapter/page numbers) is acceptable.