Beitzah 24B

שוחטין מן הנגרין ביום טוב, אבל לא מן הרשתות ומן המכמורות. רבי שמעון בן אלעזר אומר: בא ומצאן מקולקלין מערב יום טוב ⁠— בידוע שמערב יום טוב נצודו, ומותרין. בא ומצאן מקולקלין ביום טוב ⁠— בידוע שביום טוב נצודו, ואסורין.

One may slaughter animals from pens containing pools of drinking water on a Festival, but not from those found caught in nets or in traps, as they may have been caught on the Festival itself. R' Shimon ben Elazar says: If he came and found the nets and traps out of order on the eve of the Festival, which indicates that an animal had been caught in them, then it is known that the animals were caught on the eve of the Festival, and they are therefore permitted. However, if he checked the nets and traps shortly before the onset of the Festival and found them intact, and he later came and found them out of order on the Festival, it is known that the animals were caught on the Festival, and they are therefore prohibited.

הא גופה קשיא, אמרת: בא ומצאן מקולקלין מערב יום טוב ⁠— בידוע שמערב יום טוב נצודו. טעמא דבא ומצאן מקולקלין, הא ספיקא ⁠— אסורין. אימא סיפא: בא ומצאן מקולקלין ביום טוב ⁠— בידוע שביום טוב נצודו. טעמא דבא ומצאן מקולקלין, הא ספיקא ⁠— מערב יום טוב נצודו, ומותרין.

The Talmud poses a question: The baraita itself is difficult because it contains an internal contradiction between its clauses: You first said that if he came and found them out of order on the eve of the Festival, it is known that they were caught on the eve of the Festival. The reason is that he came and found them out of order, but if there is uncertainty, the animals are prohibited. But say now the latter clause of that same baraita: If he came and found them out of order on the Festival, it is known that they were caught on the Festival. The reason is that he came and found them out of order, but in a case of uncertainty, the assumption is that they were caught on the eve of the Festival and are permitted.