Axel,
The reference IS the Tipitaka itself. There, all who attained the Sotapanna stage listened. I cannot know whether during the times of previous Buddhas written (not to say printed) copies of desanas were made available, so I cannot assume that they were, but neither can the possibility be excluded.
I for one feel more at home and more enriched reading than listening, but that may be because I can ‘manage’ the content better that way : giving more time to stop and reflect on key words and phrases and so on.
During the time that I have been typing I see that Lal has replied. Still, I felt like sharing this with you.
metta