פירוש על קידושין 113:1

The Talmud now segues into a famous source about doing a midrash on the word “et.” A sage we never hear about elsewhere is said to have interpreted all of the times this word “et” appears, until he came to one whose interpretation would seem to be heretical. From that point forward, he stopped making derashot on the word et. R. Akiva, who seems to be his student, or at least his successor, succeeds in coming up with a derashah even for this verse.
We should note that we never find the word “et” being interpreted in actual tannaitic sources. Tannaim, the sages who operated in the tannaitic period, did not interpret grammatical features of verses such as full spelling and individual letters. It is only in later tradition that rabbis begin to ascribe to the tannaim such interpretive methods. This is a topic I will return to in a forthcoming third volume of Reconstructing the Talmud. So stay tuned.