Shevuot 24A

באיסור הבא מאליו; באיסור הבא על ידי עצמו לא אמרינן.

it is with regard to a prohibition that occurs on its own, like the prohibition against eating on Yom Kippur, which is more inclusive than the prohibition against eating non-kosher food and therefore takes effect. But with regard to a prohibition that occurs by the act of a person himself, i.e., an oath or a vow, we do not say that because it is more inclusive it can take effect also with regard to items that are already forbidden by Torah law.

בשלמא לריש לקיש, משום הכי קא פטר רבי שמעון; דתניא, רבי שמעון אומר: כל שהוא למכות, ולא אמרו כזית אלא לקרבן. אלא לרבי יוחנן, מאי טעמא דרבי שמעון דפטר?

The Talmud asks: Granted, according to Reish Lakish, who understands the Mishnah to be dealing with the case of one who took an oath about less than a full measure, it is due to this reason that R' Shimon deems exempt one who takes an oath prohibiting himself from eating non-kosher food, as it is taught in a baraita: R' Shimon says: Any amount is sufficient to render one liable to receive lashes, and the rabbis stated the measure of an olive-bulk to determine only liability to bring an offering. According to Reish Lakish, R' Shimon holds that one is already under oath from Mount Sinai even with regard to less than a full measure, and for that reason the oath does not take effect. But according to R' Yoḥanan, who understands the Mishnah to be referring to an oath that includes erstwhile permitted items, what is the reason that R' Shimon deems one exempt from bringing an offering for breaking his oath not to eat non-kosher food?