Yoma 80A

כל השיעורין כולן בכזית, חוץ מטומאת אוכלין, ששינה הכתוב במשמען, ושינו חכמים בשיעורן. וראיה לדבר ⁠— יום הכפורים. מאי שינה הכתוב במשמעו ⁠— מ״לא תעונה״, ומאי שינו חכמים בשיעוריה ⁠— ככותבת.

All the measures in the Torah connected to eating are the volume of an olive-bulk, except for the amount of food that renders objects impure, because the verse changed its expression in this case, and the rabbis altered the measure accordingly. The proof of this, that the rabbis gave it a different measure because the verse used different language for it, is from Yom Kippur. Also in the case of Yom Kippur the rabbis assigned a different measure because the verse used a different phrase. The Talmud asks: How did the rabbis learn that the verse changed its expression? They learned from: “Any soul which should not be afflicted” (Leviticus 23:29). The verse does not state: Any soul that should eat, but rather: “Any soul which should not be afflicted.” How did the rabbis change its measure? One does not violate the prohibition unless he has eaten the volume of a large date-bulk, as opposed to the usual olive-bulk.

ומאי ראיה לדבר יום הכפורים? דאי מהתם, הוה אמינא: אורחא דקרא הוא.

The Talmud asks: And what does the baraita mean when it says: A proof for this is from Yom Kippur? Why is the verse pertaining to ritual impurity not sufficient to show that the rabbis changed the measure based on the different words in the verse? The Talmud answers: If we learned it only from there, the case of impurity, I would have said that that is the style of the verse, and no halakha can be derived from it. Therefore, the verse pertaining to Yom Kippur teaches that whenever a verse deviates from the usual language, it implies a change in the halakha as well.