Augustine describes silent reading himself:
“I returned in haste to the place where Alipius was sitting, for I had left the book of the Apostle there when I stood up. I seized it, opened it, and read in silence the chapter upon which my eyes first fell”
https://sourcelibrary.org/book/meditationes-divi-augustini-e...
Using the SourceLibrary MCP in Claude, I also found an example going all the way back to Euripides. In this scene, Theseus reads a tablet and the chorus has to ask what it says (because he read it, but not out loud).
https://sourcelibrary.org/book/euripides-i-ion-hippolytus-me...