Reference for Shabbat 297:7
היכי דמי אילימא דכתיב מיכתב מ"ש הכא ומ"ש הכא אלא לאו דחייק וקתני מכתב שעל גבי הכותל אבל לא מכתב שע"ג טבלא ופינקס
and how many portions he is to set before them, from writing on a wall, but not from writing on a tablet or a board. How is it meant? Shall we say that it is indeed written, wherein does one differ from the other? Hence it must surely mean that it is engraved, yet he states, 'from writing on the wall, but not from writing on a tablet or a board'? — Rather [say thus]: In truth [they differ] where it is written high up on the wall, and as for your difficulty about Rabbah's [ruling], [the ruling] of Rabbah is dependent on Tannaim. For it was taught: A man may count his guests and his dainty portions by word of mouth, but not from writing. R. Aha permits [it] from writing on the wall. How is it meant: Shall we say that it is written low down, — then let us fear lest he erase it? Hence it must surely mean that it is written high up, which proves that Rabah's [ruling] is dependent on Tannaim.