Gemara

63a: Ravina saw a man transgressing. He shouted at him, but the man did not stop. Ravina excommunicated him. (Rav Ashi was there.)

Ravina: Was it brazen for me to do so in front of you?

Rav Ashi: "Ein Chachmah... l'Neged Hash-m" - we do not stop to honor the Rebbi when Hash-m's name is being desecrated.

67b: A case occurred in which the hot water prepared for Bris Milah spilled. Rabah told people 'bring hot water from my house'!

Abaye: There is no Eruv [in the Chatzer]!

Rabah: Tell a Nochri to bring the water from my house.

Abaye: I was about to ask a question. Rav Yosef stopped me;

(Rav Yosef): If a Chacham permits something in practice, and a Talmid thinks that it is forbidden mid'Oraisa, the Talmid asks before they do it;

If he thinks that it is forbidden mid'Rabanan, he asks after they do it.

Kidushin 32a (Beraisa): If one sees his father transgress a Torah law he says 'father, there is a verse in the Torah that says...'

Chulin 6a: R. Zeira and Rav Asi lodged at an inn. They were served eggs that had been shriveled in wine. R. Zeira did not eat; Rav Asi ate.

R. Zeira: Aren't you concerned lest they absorbed Demai (doubtfully tithed) wine?

Rav Asi: I did not think about that.

R. Zeira thought, if it is forbidden, could Rav Asi mistakenly eat it? Hash-m does not allow a pitfall to come through Tzadikim, and not even through their animals!

R. Zeira found a Mishnah that exempts one who bought a mixture containing Demai from tithing it.

6b: R. Yehoshua ben Zeruz testified that R. Meir ate a vegetable leaf from Beis She'an without tithing. Rebbi exempted Beis She'an from Ma'aseros.

7a - Question (R. Yirmeyah): Perhaps R. Meir forgot to tithe it!

Rejection (R. Zeira): Hash-m does not allow a pitfall to come through Tzadikim!

30b - R. Zeira (to Rav Yitzchak): Rebbi, didn't you teach to us that the Mishnah permits Shechitah done by two people [only] when they hold one knife?!

Berachos 16b (Mishnah): R. Gamliel bathed on the night after his wife died.

His Talmidim: You taught us that a mourner may not bathe!

R. Gamliel: I am an exception. I am an Istenis (very sensitive).

When R. Gamliel's slave Tavi died, he accepted consolation.

His Talmidim: You taught us that we do not accept consolation for slaves!

R. Gamliel: Tavi was not a regular slave. He was proper.

Rosh Hashanah 29b (Beraisa): Once, Rosh Hashanah was on Shabbos. R. Yochanan ben Zakai told Bnei Beseirah to blow the Shofar [in Yavneh. The Sanhedrin was in exile there.]

Bnei Beseirah: Let us discuss this!

R. Yochanan ben Zakai: Blow, and afterwards we will discuss it.

Afterwards, they asked to discuss it.

R. Yochanan ben Zakai: The Shofar was already heard. We do not question after the act.

Kesuvos 69a: Rav Anan sent to Rav Huna 'Huna our colleague...' (Rav Huna was a much greater Chacham.)

Rav Huna (to Rav Sheshes): Say the following [impolite message] to Rav Anan verbatim! If not, you will be excommunicated! 'Anan, Anan!...'

Rav Sheshes delivered the message verbatim, but first apologized to Rav Anan.

Rishonim

Rambam (Hilchos Talmud Torah 5:9): If one sees his Rebbi transgress Divrei Torah, he says 'Rabbeinu, you taught to us such and such.' Whenever one says a teaching in front of his Rebbi, he says 'so you taught to us.'

Kesef Mishneh: The Gemara says that one does not say to a father 'you transgressed.' The Rambam learns that all the more so, this applies to one's primary Rebbi.

Maharatz Chayos (30b DH Tosfos): Our Gemara is a proof for the Rambam, that also when correcting one's Rebbi one must say 'you taught to us.'

Terumas ha'Deshen (43): If one sees that his Rebbi wants to eat something forbidden mid'Rabanan, should he protest? One of my Rebbeyim said that he need not. He learns from Chulin 6a. R. Zeira asked only after Rav Asi ate. This shows that one need not protest. I say that perhaps this is only when also the Talmid is unsure if there is an Isur. R. Zeira later proved that it is permitted! If he knows that it is forbidden, even mid'Rabanan, he must inform him and separate him [from the Isur]. Rashi there says "since the Demai was not intact, they were not concerned for it.' This implies that Rav Asi and R. Zeira were not concerned. R. Zeira did not eat! Rather, he was not concerned to protest. Had it been intact, he would have protested, even though the Isur is only mid'Rabanan. Perhaps R. Zeira relied on this not to protest about a Safek.

Terumas ha'Deshen: We cannot learn from here that one need not protest about a Safek nowadays. Tosfos (Chulin 5b DH Tzadikim) brings from Bereishis Rabah that R. Yirmeyah sent a basket of fruits to R. Zeira. In the meanwhile, they were eaten while Tevel. R. Zeira said about this (Shekalim 13b) "if the early Chachamim were like people, we are like donkeys.' If a pitfall occurred to Tzadikim in his days, all the more so we must be concerned nowadays! Also, Tosfos says that Hash-m does not allow a pitfall of eating Isur to occur to or through Tzadikim. (R. Zeira relied on this not to protest.) Other Isurim, even a Tzadik himself could stumble in. E.g. R. Yishmael read [by light of a Ner on Shabbos] and tilted it. If so, one who sees his Rebbi about to transgress such a matter must protest, even if we would equate nowadays to earlier generations.

Terumas ha'Deshen: Eruvin 67b does not prove that one need not separate from an Isur mid'Rabanan. For an Isur mid'Rabanan, one asks after they do it. Rashi explains that if a Talmid saw the Rebbi permit an Isur mid'Rabanan, and he can challenge it, he lets the action be done, and asks afterwards. Sefer ha'Terumah agrees. When the Talmid is unsure if it is an Isur mid'Rabanan, he does not enter his Rebbi's action until afterwards. We cannot learn from there to here [when the Rebbi is about to eat]. There, the Rebbi ruled about a Halachah in practice. Presumably, he considered it well to rule properly. Even if the Talmid thinks that he can challenge it, he waits until afterwards. Perhaps the Rebbi has an answer. If one sees his Rebbi about to eat something, and there is concern for Isur, he need not be quiet. He should think that the Rebbi is unaware, and forgot. The Gemara asked 'perhaps R. Meir forgot. Why did Rebbi permit through this testimony?' Had one person testified that R. Meir ruled to permit Peros of Beis She'an without tithing, surely Rebbi would` rely on it, and we would not have asked anything. In any case, we find a proof in two places (Eruvin and Chulin 6a). Even though we rejected the proofs in different ways, one need not be so meticulous.

Poskim

Shulchan Aruch (YD 242:22): If one sees his Rebbi transgress Divrei Torah, he says 'Rabbeinu, you taught to us such and such.'

Shach (42): He must protest because he is sure that he transgresses. Terumas ha'Deshen was unsure if he must protest due to Safek. Eruvin discusses one who rules in practice. This is unlike a Rebbi about to eat Isur. This seems primary, but the Rema follows Terumas ha'Deshen's conclusion not to be so exacting.

Gra (58): Terumas ha'Deshen was unsure about this. He rejected the proof from Eruvin. There he is unsure if the Rebbi will transgress. The law is clear from Berachos 16b. Tosfos says that all Avelus is mid'Rabanan. Similarly, we do not accept consolation for slaves [mid'Rabanan, yet the Talmidim questioned R. Gamliel].

Erech Lechem: One may say incredulously 'is this the Halachah?!' (Shabbos 45a, Bava Metzi'a 83a). One may not say 'this law is not so!' in astonishment, opposite to the Rav (Sanhedrin 101a). A Talmid may not transgress his Rebbi's Cherem, even if it is not totally justified (Kesuvos 69a). Rav Sheshes could have protested for the honor of his Rebbi in another way, without saying "Anan, Anan." One should not be so meticulous to protest against his Rav when he transgresses an Isur mid'Rabanan.

Yom Teru'ah (Rosh Hashanah 29b DH Amar): R. Yochanan ben Zakai responded "the Shofar was already heard. We do not question after the act." What does this mean? For mid'Rabanan laws, we ask after the act (Eruvin 67b)! The Rema rules like this. I answer that in Eruvin, there is a Vadai question. We ask to find the answer. Here, it was not clear whether the previous decree applied to the Beis Din in Yavneh. R. Yochanan ben Zakai said that once they blew, we do not debate about a decree. If most of the Sanhedrin will say to decree, it will be disgraceful that they did improperly. In such a case, we do not question after the act.

B'Tzel ha'Chachmah (3:20): Does a Talmid say 'you taught to us' even if the Rebbi is about to transgress something that he did not learn from him? Just like we deviate [from the truth] for the sake of Shalom, also for the Rebbi's honor. The Rambam connotes like this, for he did not distinguish. Or, perhaps one says 'the verse says...', like one says to his father. The Kesef Mishneh says that the Rambam learned from correcting one's father. The Ritva (in Shitah Mekubetzes Bava Metzi'a 31a) says to say "you taught to us", like it says in Kidushin. Why didn't the Ritva and Kesef Mishneh learn from Berachos 16b, which explicitly discusses a Rebbi? Why didn't the Terumas ha'Deshen prove from there that one must rebuke a Rebbi for an Isur mid'Rabanan? The Gra alludes to this. Surely, even if one need not protest at the time, he must ask afterwards, like in Chulin 6a and Eruvin 67b. Surely, they asked R. Gamliel after he bathed. They could not ask while he bathed, for a Talmid may not bathe with his Rebbi (Pesachim 51a).

Note: Perhaps he asked them to bring his clothes to the bathhouse, and they asked then! However, the words of the Mishnah connote that they asked after he bathed.

B'Tzel ha'Chachmah: Therefore, we cannot bring a proof from here. The Ritva and Kesef Mishneh did not learn from there, for perhaps then 'you taught us' suffices. They discuss separating a Rebbi from a Torah Isur before he does it. This is more severe. Perhaps one must explicitly state the severity of the Isur, and not rely on the Rebbi to realize that he erred. The Hafla'ah (Chidushim on CM 18:1) says so about "Lo Sa'aneh Al Rav." In capital cases, one must explicitly dispute the ruling, and not merely question it. We could not do so if the Gadol would initially say that the person is liable. Also, perhaps R. Gamliel's Talmidim said "you taught to us" to show extra honor, but this is not obligatory. If a Talmid did not learn the law from his Rebbi, and it is not explicit, just it seems that it is forbidden, he says 'the master is not concerned for [or does not hold] that...?'