Avodah Zarah 74A

מתני׳ אלו אסורין, ואוסרין בכל שהו: יין נסך, ועבודה זרה, ועורות לבובין,

Mishnah: These following items are themselves forbidden, and any amount of them renders other items with which they become mixed forbidden: Wine used for a libation; and objects of idol worship; and hides with a tear opposite the heart, indicating the idolatrous practice of sacrificing hearts of live animals.

ושור הנסקל, ועגלה ערופה,

And this halakha also applies to an ox that has been condemned to be stoned (see Exodus 21:28), from which it is prohibited to derive benefit even before its sentence is carried out; and it applies to a heifer whose neck is broken when a person is found killed in an area between two cities and the murderer is unknown (see Deuteronomy 21:1–9), which is likewise forbidden from the time it is taken down to the river to be killed. In these cases, if the animal becomes mixed in a herd of similar animals, all of the animals in the herd are forbidden.

וציפורי מצורע, ושער נזיר, ופטר חמור, ובשר בחלב, ושעיר המשתלח, וחולין שנשחטו בעזרה ⁠— הרי אלו אסורין ואוסרין בכל שהוא.

And this halakha also applies to birds designated for the purification of a metzora (Leviticus 14:1–6), and the shorn hair of a nazirite (Numbers 6:18), and a firstborn donkey (Exodus 13:13), and meat that was cooked in milk (Exodus 23:19), and the scapegoat of Yom Kippur (Leviticus 16:7–10), and the meat of a non-sacred animal that was slaughtered in the Temple courtyard. All of these are forbidden themselves, and any amount of them renders a mixture forbidden.

גמ׳ תנא מאי קחשיב? אי דבר שבמנין קחשיב, ליתני נמי חתיכות נבילה! אי איסורי הנאה קא חשיב, ליתני נמי חמץ בפסח! אמר רבי חייא בר אבא, ואיתימא רבי יצחק נפחא: האי תנא תרתי אית ליה ⁠— דבר שבמנין ואיסורי הנאה.

Talmud: According to what criterion does the tanna who teaches this Mishnah reckon cases? If he reckons based on any item that is counted, i.e., any item that is significant enough to be considered individually, which therefore cannot be nullified in a mixture even in a very large majority of permitted items, let him also teach the case of significant cuts of an unslaughtered animal carcass. And if he reckons based on items from which deriving benefit is prohibited, let him also teach the case of leavened bread on Passover. R' Ḥiyya bar Abba said, and some say it was R' Yitzḥak Nappaḥa who said: This tanna has two criteria. He reckons based on any item that is both counted and from which deriving benefit is prohibited.