It is often repeated on various "build up your wealth" guides as the "1% rule". here is a random link I found by searching, but there are many others, that directly states "The 1% rule says: monthly rent should be at least 1% of purchase price. On a $200,000 property, you'd want monthly rent of $2,000+."
We clearly head different things about "landlording". My knowledge of landlording is mostly from online references suggesting how to do it well, and the remaining if horror stories from associates who didn't do any research.