Shabbat 78A

וליתא דרבי ינאי. והכא בהא קמיפלגי: רבי שמעון בן אלעזר סבר: אבר קטן דגדול ואבר גדול דקטן בן יומו כי הדדי נינהו, ורבי נתן סבר: אבר קטן דגדול אין, אבר גדול דקטן בן יומו ⁠— לא. מאי הוי עלה? תא שמע: דתניא, רבי שמעון בן אלעזר אומר: שמן כדי לסוך אבר קטן של קטן בן יומו.

and the opinion of the school of R' Yannai is not accepted. And here, in this baraita, they disagree about this: R' Shimon ben Elazar holds: A small limb of an adult and a large limb of a day-old child are equal to one another, and R' Natan holds: For carrying out oil in a measure equivalent to that which is used to spread on a small limb of an adult, yes, one is liable; however, a large limb of a day-old child, no, he is exempt. The Talmud asks: What conclusion was reached in this matter? Come and hear a proof, as it was taught in a baraita that R' Shimon ben Elazar says explicitly: The measure that determines liability for carrying out oil is equivalent to that which is used to spread on a small limb of a day-old child.

מים כדי לשוף בהן את הקילור. אמר אביי: מכדי כל מילתא דשכיחא ולא שכיחא, אזול רבנן בתר דשכיחא לקולא. שכיחא ושכיחא אזול רבנן בתר דשכיחא לחומרא.

We learned in the Mishnah: The measure that determines liability for carrying out water is equivalent to that which is used to rub and spread on an eye bandage. Abaye said: Now, since, with regard to any substance that is utilized for both common and uncommon uses, the rabbis, in their ruling, followed the common usage even as a leniency, i.e., one is liable only for carrying out the larger measure. However, when a substance has different uses and one is common and the other is common as well, the rabbis, in their ruling, followed the common use that leads to a stringency, i.e., one is liable for carrying out even the smaller amount.