Beitzah 16B

הרסנא עיקר, קא משמע לן קמחא עיקר.

the fish fat, which one is permitted to eat even if cooked by non-Jews, is the essential element. Therefore, he teaches us that the flour is the essential element, and the non-Jew has created a new and significant dish, which is consequently prohibited.

אמר רבי אבא: ערובי תבשילין צריכין כזית. איבעיא להו: כזית אחד לכלן, או דלמא כזית לכל אחד ואחד? תא שמע, דאמר רבי אבא אמר רב: ערובי תבשילין צריכין כזית בין לאחד, בין למאה.

R' Abba said: A joining of cooked foods requires an olive-bulk of food. A dilemma was raised before the rabbis: Is it enough to have one olive-bulk for all of the household members who are relying on this eiruv, or perhaps must there be a separate olive-bulk for each and every one in the household? Come and hear that which R' Abba said that Rav said: A joining of cooked foods requires an olive-bulk, whether it is cooked for a single person or for 100 people.

תנן: אכלו או שאבד ⁠— לא יבשל עליו בתחלה, שייר ממנו כל שהוא ⁠— סומך עליו לשבת. מאי כל שהוא? לאו אף על גב דליכא כזית? לא, דאית ביה כזית.

The Talmud comments: We learned in the Mishnah: If one ate the joining of cooked foods that he prepared, or if it was lost, he may not rely on it and cook with the initial intent to cook for Shabbat; but if he left any part of the eiruv, he may rely on it to cook for Shabbat. The Talmud asks: What is the meaning of: Any part of it? Doesn’t it mean that it is valid although there is not an olive-bulk? The Talmud answers: No, the phrase: Any part of it, means that there is at least an olive-bulk left.

תא שמע: תבשיל ⁠— זה צלי, ואפילו כבוש שלוק ומבושל, וקולייס האספנין שנתן עליו חמין מערב יום טוב ⁠— תחלתו וסופו אין לו שיעור. מאי לאו, אין לו שיעור כלל? לא, אין לו שיעור למעלה, אבל יש לו שיעור למטה.

The Talmud attempts to bring another proof. Come and hear: The cooked dish that is required for a joining of cooked foods may be roasted, or even pickled, or well-boiled, or boiled in the regular manner, or may even be Spanish soft sea fish [koleyas ha’ispenin] upon which one poured hot water on the eve of the Festival, rendering it cooked. With regard to its beginning and end, meaning its required size ab initio and after it has been reduced by being partially lost or partially eaten, the eiruv has no required measure. What, is it not that it has no required measure at all, not even a minimum one? The Talmud rejects this: No, it means it does not have a maximum measure, but it does have a minimum measure, namely an olive-bulk.