פירוש על עבודה זרה 152:4

'What you did at night' -- It was the way of the Persians to give women to their guests, and when they sent [the women] to them the previous night, Bati received [the woman], but Rav Yehudah did not recieve [the woman].

In this fascinating story, the Persian King Shappur cuts some etrog slices and gives it Bati b. Tobi without kashering his knife, but kashers the knife before he gives it to Mar Yehudah. Bati is not too happy that he is not being treated like a non-Jew. The king responds that he does not know if Bati b. Tobi really is observant. And in a highly humorous end to the masekhet, the king says to him, “I remember what you did last night.” Too bad we’ll never know what naught Bati did last night. And come on, can you think of a better way to end Masechet Avodah Zarah then with a non-Jewish king kashering a knife and telling another Jew he is not observant enough?