Yoma 74A

ואליבא דרבי עקיבא דאמר: אדם אוסר עצמו בכל שהוא!

and the Mishnah is in accordance with the opinion of R' Akiva, who said: A man prohibits himself from any amount. If a man swears that he will not eat, he thereby prohibits himself from eating even the smallest amount of food. Therefore, Reish Lakish himself maintains that eating a half-measure does not constitute a prohibition.

וכי תימא: כיון דאית ליה היתר מן התורה, קא חייל קרבן שבועה, והתנן: שבועת העדות אינה נוהגת אלא בראויין להעיד. והוינן בה: למעוטי מאי? רב פפא אמר: למעוטי מלך.

And if you say: Perhaps Reish Lakish maintains that since a half-measure is permitted by Torah law, despite the fact that it is prohibited by rabbinic law, one is liable to bring an offering for violating an oath, then there is the following problem: Didn’t we learn in a Mishnah: An oath of testimony where one is sworn to give testimony on something that he saw or knew, applies only to those who are eligible to give testimony. If one who is ineligible to testify swears an oath to give testimony, the oath is invalid even if he does not testify. And we discussed it: The statement: Those who are eligible to give testimony, comes to exclude what? After all, it was already said that the oath does not apply to women, relatives, and other disqualified people. Rav Pappa said: It comes to exclude a king. A king is not disqualified from giving testimony, but he does not testify before a court, due to the requirement to give respect to a king.

רב אחא בר יעקב אמר: למעוטי משחק בקוביא. והא משחק בקוביא ⁠— מדאורייתא מיחזי חזי, ורבנן הוא דפסלוהו, ולא קא חיילא עליה שבועה!

Rav Aḥa bar Ya’akov said: It comes to exclude one who plays with dice, whom the rabbis disqualified from giving testimony. But surely one who plays with dice is eligible by Torah law to give testimony, and it is the rabbis who disqualified him. Despite this, an oath of testimony does not apply to him by Torah law, even though the prohibition on his testifying is rabbinic.