This doesn't sound like the sort of thing for which it is meaningful to have a "first guy that said this" reference for, unless you are writing specifically about the history and development of modern coding standards. And even then it's difficult or impossible to find a single, definitive source. More than likely the ideas emerged dynamically from a whole host of forces and in multiple people/locations, rather than any one person who "said it first" and everyone else followed the lead of. What is "good" and "quality" is largely a matter of convention and opinion, after all.
Having hundreds of textbooks and thousands of research articles that support it is more than sufficient to establish it as a common mantra, or whatever you want to call it. If you can find a respectable book that is specifically about modern coding practice and its (historical) development, you could reference that as your support for the claim. Otherwise referencing a few major, influential texts that make the same claims would be good, though you may want to be careful with your phrasing. Citing them as proof this is the one "right" or "universal" way to do coding is arguably misleading and circular; saying that you take the same approach as those authors, however, makes no claims to objective truth and simply makes clear an assumption to the reader.
