Who is the author of the phrase "code is read much more often than it is written" and where was it mentioned for the first time?

I want to use that quote in an university paper and I want to include the correct reference. I'm almost sure that it was Guido Von Rossum since is mentioned in the PEP8 text:

One of Guido's key insights is that code is read much more often than it is written. The guidelines provided here are intended to improve the readability of code and make it consistent across the wide spectrum of Python code. As PEP 20 says, "Readability counts".

But it seems to refer to a previous document and I couldn't find which one.

I also suspect that maybe it has been a widely accepted idea during the last decades of software development and doesn't have a clear author