Beitzah 26B

ביום טוב, רבי שמעון אומר: אין זה מן המוכן. ושוין שאם נולד הוא ומומו עמו ⁠— שזה מן המוכן.

on the Festival itself, R' Shimon says: It is not considered to be among the animals prepared prior to the Festival for use on the Festival; therefore, even if it was examined by an expert, it may not be slaughtered. And R' Yehuda and R' Shimon agree that if the animal was born with its blemish, it is considered to be among the animals prepared for use on the Festival.

דרש רבה בר רב הונא: נולד הוא ומומו עמו ⁠— מבקרין אותו ביום טוב לכתחלה. אמר ליה רב נחמן, אבא תני: אם עבר ובקרו ⁠— מבוקר, ואת אמרת מבקרין אותו לכתחלה!

Rabba bar Rav Huna taught the halakha as follows: If a firstborn was born with its blemish, experts may examine it on a Festival ab initio, and if the blemish is found to be permanent, the animal may then be slaughtered. Rav Naḥman said to him: Father would teach on this matter that if he transgressed and examined it, it is considered examined after the fact, and you say that one may examine it even ab initio?

אמר אביי: כותיה דרבה בר רב הונא מסתברא, מדקתני תלתא בבי: נולד בו מום מערב יום טוב ⁠— אין מבקרין אותו ביום טוב. לכתחלה הוא דלא, הא דיעבד ⁠— שפיר דמי.

Abaye said: It stands to reason that the halakha is in accordance with the opinion of Rabba bar Rav Huna from the fact that the baraita is taught in 3 parts, which suggests that each part contains a novel halakha. The first section states: If a firstborn developed a blemish on the eve of a Festival one may not examine it on the Festival itself, from which one can learn that it is only ab initio that one may not examine it; but after the fact, it seems well, and the examination is valid.

נולד בו מום ביום טוב, רבי שמעון אומר: אין זה מן המוכן ⁠— דאפילו דיעבד נמי לא. והדר תני: ושוין שאם נולד ומומו עמו, שזה מן המוכן ⁠— דאפילו לכתחלה נמי.

The second section of the baraita states: If it developed a blemish on the Festival itself, R' Shimon says: It is not considered to be among the animals prepared prior to the Festival for use on the Festival. In other words, even after the fact, no, if the animal was examined by an expert, it may not be slaughtered and eaten. And then in the third section it teaches: And R' Yehuda and R' Shimon agree that if the animal was born with its blemish, it is considered to be among the animals prepared for use on the Festival. The fact that the baraita taught this halakha as an independent clause indicates that even ab initio as well, the animal may be examined.