Avodah Zarah 35A

מכלל דאיסורי הנאה שרו פרשייהו.

One can learn by inference from here that with regard to animals from which deriving benefit is prohibited, their excrement, which is the content of their stomach, is permitted. Although deriving benefit from both a burnt-offering and an unslaughtered animal carcass is prohibited, the excrement of each is permitted. Similarly, although deriving benefit from an ox that is to be stoned is prohibited, its excrement is permitted.

ומדקאמר ליה מפני שמעמידין אותה בקיבת עגלי עבודה זרה, וקא מהדר ליה: אם כן למה לא אסרוה בהנאה ⁠— מכלל דעבודה זרה אסיר פרשייהו.

And from the fact that R' Yehoshua said to R' Yishmael: Cheese of non-Jews is prohibited because they curdle it with the stomach contents of calves used for idol worship, and that R' Yishmael responded to him: If that is so, why didn’t the rabbis prohibit deriving benefit from the cheese, one may learn by inference that with regard to animals of idol worship, their excrement is prohibited. Since the cheese formed with the stomach contents of an animal of idol worship is prohibited, it is evident that the excrement formed in the stomach of such an animal is also prohibited.

ולהדר ליה, משום דליתיה לאיסורא בעיניה!

The Mishnah related that rather than addressing R' Yishmael’s final difficulty, R' Yehoshua diverted his attention to another matter. The Talmud inquires: But let him respond to R' Yishmael’s query by explaining that the rabbis did not prohibit deriving benefit from cheese curdled in the stomach contents of an animal used for idolatry because there is no substantive prohibited entity in such cheese.

דהא מורייס, לרבנן דלא אסרוה בהנאה, מאי טעמא? לאו משום דליתיה לאיסורא בעיניה?

The Talmud reinforces its question: After all, isn’t the halakha with regard to fish stew, according to the opinion of the Rabbis, an application of this rationale, as they did not prohibit deriving benefit from fish stew prepared by a non-Jew? What is the reason for this leniency? Is it not because there is no substantive prohibited entity in it? Although fish stew may contain the wine of a non-Jew, deriving benefit from it is not prohibited because the wine is not discernible. Why didn’t R' Yehoshua explain that deriving benefit from cheese of a non-Jew is similarly permitted because it contains no substantive prohibited entity?