ר' זירא כי הוה חליש מגירסיה הוה אזיל ויתיב אפתחא דבי ר' נתן בר טובי אמר כי חלפי רבנן אז איקום מקמייהו ואקבל אגרא נפק אתא ר' נתן בר טובי א"ל מאן אמר הלכה בי מדרשא א"ל הכי א"ר יוחנן אין הלכה כר' יהודה דאמר מתפלל אדם של מוסף ואח"כ מתפלל של מנחה א"ל רבי יוחנן אמרה אמר ליה אין תנא מיניה ארבעין זמנין א"ל חדא היא לך או חדת היא לך א"ל חדת היא לי משום דמספקא לי בר' יהושע בן לוי:
Or, in English:
When R. Zera was tired from studying, he used to go and sit by the door of the school of R. Nathan b. Tobi. He said to himself: When the Rabbis pass by, I will rise before them and earn a reward.31 R. Nathan b. Tobi came out. He said to him: Who enunciated a halachah in the Beth ha-Midrash? He replied: Thus said R. Johanan: The halachah does not follow R. Judah who said that a man first says the musaf Tefillah and then the minhah one. He said to him: Did R. Johanan say it? — He replied, Yes.32 He repeated it after him forty times. He said to him: Is this the one [and only] thing you have learnt [from him]33 or it is a new thing to you?34 He replied: It is a new thing to me, because I was not certain [whether it was not the dictum] of R. Joshua b. Levi.Just in passing, חדא היא לך או חדת היא לך might indeed be an idiom, but it strikes me that it might be a variant girsa encoded in the Talmudic text, where it was unclear whether chada or chadat had been written. Still, chada vs. chadat also sounds like wordplay.
The standard understanding of this gemara is that Resh Lakish was fully aware of this statement, but he was simply uncertain whether it was Rabbi Yochanan or Rabbi Yehoshua ben Levi who had said it.
I would suggest a different interpretation. The Mishna had recorded this dispute between Rabbi Yehuda and the Chachamim as to the time of Musaf, with Rabbi Yehuda saying until the seventh hour and the Chachamim saying that the whole day was fine. We pasken like the Chachamim in this. Now, there is another dispute between Rabbi Yehuda and the Chachamim as to which comes first, mincha or mussaf, it the time arises where both are available. To cite the immediately preceding gemara:
ת"ר היו לפניו שתי תפלות אחת של מנחה ואחת של מוסף מתפלל של מנחה ואח"כ מתפלל של מוסף שזו תדירה וזו אינה תדירה ר' יהודה אומר מתפלל של מוסף ואח"כ מתפלל של מנחה שזו מצוה עוברת וזו מצוה שאינה עוברת א"ר יוחנן הלכה מתפלל של מנחה ואח"כ מתפלל של מוסף
Our Rabbis taught: If a man had two Tefillahs to say, one for minhah and one for musaf, he says the one for minhah, and afterwards he says the one for musaf. because the one is daily29 and the other is not daily. R. Judah says: He says the musaf one first and then he says the minhah one; the former is an obligation that will soon lapse30 while the other is an obligation that will not lapse.If we consider for a bit, we will realize that the reason for calling musaf a mitzvah overet, an obligation that will soon lapse, is only in accordance with Rabbi Yehuda, who sets a specific time for Mussaf. The Chachamim might have agreed in principle with the idea of mitzvah overet, but here, mussaf is no mitzvah overet.
If so, it should be obvious that we do not pasken like Rabbi Yehuda here. If so, why should it be a surprise that Rabbi Yochanan would be the one to say it? Because Rabbi Yochanan enunciated the same principle as Rabbi Yehoshua ben Levi.
That is, in the gemara which immediately follows, Rabbi Yehoshua ben Levi stated:
אריב"ל כל המתפלל תפלה של מוספין לאחר שבע שעות לר' יהודה עליו הכתוב אומר (צפניה ג, יח) נוגי ממועד אספתי ממך היו מאי משמע דהאי נוגי לישנא דתברא הוא כדמתרגם רב יוסף תברא אתי על שנאיהון דבית ישראל על דאחרו זמני מועדיא דבירושלים
R. Joshua b. Levi said: If one says the musaf Tefillah after seven hours, then according to R. Judah the Scripture says of him, I will gather them that are destroyed [nuge]35 because of the appointed season, who are of thee.36 How do you know that the word 'nuge' here implies destruction? It is as rendered by R. Joseph [in his Targum]:37 Destruction comes upon the enemies of Israel38 because they put off till late the times of the appointed seasons39 in Jerusalem.Thus, even though we don't absolutely pasken like Rabbi Yehuda in the Mishna, we do hold like him a bit, as a lechatchila, that if someone lets the time pass it is a very negative thing. And earlier in the gemara, Rabbi Yochanan gave voice to the same thought, saying:
ושל מוספין כל היום:א"ר יוחנן ונקרא פושע
THE TIME FOR THE ADDITIONAL PRAYER IS THE WHOLE DAY. R. Johanan said: And he is [nevertheless] called a transgressor.28If Rabbi Yochanan signs on to the idea also expressed by Rabbi Yehoshua ben Levi, that there is a rather important value to the time set by Rabbi Yehuda, then perhaps he would call that time a mitzvah overet! And if so, then we should think that Rabbi Yochanan might well pasken like Rabbi Yehuda in this brayta, and declare that one should daven mussaf first, since it is a mitzvah overet!
And this is the meaning of Resh Lakish's seizing upon the fact that it was Rabbi Yochanan who said it. And this is the meaning of his statement that חדת היא לי משום דמספקא לי בר' יהושע בן לוי.