I have often heard or read the expression, "code is written once but read many times". I think this is a fairly well-known principle of software development. But I have no idea with whom to credit with the notion.
Its not hard to find references which use it, but I don't think I've ever seen a citation or credit given. It could be someone well known (e.g., Donald Knuth? Douglas Hofstadter? "Gang of Four"?) or less well known but deserving recognition for the insight.
Examples of usage:
Code is read much more often than it is written, so plan accordingly...
...the ratio of time spent reading versus writing is well over 10 to 1. We are constantly reading old code as part of the effort to write new code.
Wording varies, but its the same idea.
I guess its possible that the original statement of this idea predates computer programming. Artifacts such as cooking recipes, circuit diagrams, architectural plans, etc. probably share the same principle. But still, someone had to eventually recognize the application to source code and write about it or otherwise popularize the idea.