Shevuot 4A

אינו חייב אלא אחת. זו היא שבועת ביטוי, שחייבין על זדונה מכות, ועל שגגתה קרבן עולה ויורד.

he is liable for only one violation. Since the second oath did not prohibit any act in addition to the first oath, he is not liable for violating it. The Mishnah concludes: This is an oath on an utterance for which one is liable to receive lashes for its intentional violation, and to bring a sliding-scale offering for its unwitting violation.

״זו היא דחייבין על זדונה מכות״⁠ – אבל ״אוכל״ ולא אכל, לא לקי!

The Talmud infers from the formulation of the Mishnah: It is specifically this case of an oath on an utterance for which one is liable to receive lashes for its intentional violation. But if a person stated: On my oath I will eat, and then he did not eat, he is not flogged. Presumably this is because the violation does not involve any action. This Mishnah, then, can provide a basis for R' Yoḥanan’s ruling.

מכדי האי סתמא והאי סתמא; מאי חזי דעביד כי האי סתמא? לעביד כי האי סתמא!

The Talmud asks: Now, this Mishnah (27b) is unattributed and that Mishnah (2a) is unattributed. What did R' Yoḥanan see that he practiced in accordance with this unattributed Mishnah? Let him instead practice in accordance with that unattributed Mishnah.

ולטעמיך ⁠– רבי גופיה, היכי סתם לן הכא הכי והכא הכי?

The Talmud adds another question: And according to your reasoning that both mishnayot carry equal weight, then with regard to R' Yehuda HaNasi himself, when he redacted the Mishnah, how could he teach us as unattributed both this opinion in the Mishnah here and that opinion in the Mishnah there? This would result in a contradiction.

אלא מעיקרא סבר לאו שאין בו מעשה לוקין עליו, וסתמה, והדר סבר אין לוקין עליו, וסתמה; ומשנה לא זזה ממקומה.

Rather, it is apparent that initially R' Yehuda HaNasi held that for a violation of a prohibition that does not involve an action one is flogged, and so he taught it as an unattributed Mishnah, and later he retracted his opinion and held that one is not flogged for a violation of such a prohibition, and so he taught that opinion as an unattributed Mishnah. And he left the first Mishnah (2a) as it was because a Mishnah does not move from its place, i.e., since the students had already learned that Mishnah it was deemed inappropriate to remove it. Based on this logic, R' Yoḥanan ruled in accordance with the later Mishnah (27b).