Gemara

Question (R. Shimon ben Elyakim): Why does R. Meir say that one who buys one tree brings Bikurim but does not recite, and why do Chachamim say so about one who buys two trees?

R. Elazar: The reason was never explained. How should I know?! Your question embarrasses me!

Chulin 6a (R. Chiya - Beraisa): "When you sit to combat the ruler (to learn from your Rebbi), contemplate what is in front of you. Put a knife in your cheek, if you value your soul" - ask a question only if you think that your Rebbi can answer. If not, refrain from asking.

Bava Metzia 58b (Beraisa): If donkey-drivers ask one for grain, he may not direct them to one whom he knows does not sell.

Horayos 13b: R. Meir suggested to R. Noson that they ask R. Shimon ben Gamliel to teach Uktzim (R. Shimon did not know Uktzim. This would be grounds to depose him. R. Noson would become the Nasi, and R. Meir would become the Av Beis Din.) R. Yakov ben Kodshi heard this. He was concerned lest R. Shimon be shamed. He hinted to him that he should learn Uktzim.

Shabbos 3b - Question (Rav): If Reuven loaded food and drink on Shimon's body, and Shimon went outside (to Reshus ha'Rabim), what is the law? Is Akirah of his own (laden) body like Akirah of an object?

Answer (Rebbi): He is liable. Akirah of his body is not like Akirah of his hand.

R. Chiya (to Rav): You should only ask about the tractate your Rebbi is teaching. If Rebbi were not a great Chacham, he might have answered incorrectly, and he would have been embarrassed! (Fortunately,) Rebbi is a great Chacham, and he answered well.

114a (R. Yochanan): A Chacham is qualified to be appointed over the Tzibur if people can ask him a Halachah in any place and he can answer, even 'Kalah.'

(R. Yochanan): A Chacham is one who can answer a question about any Halachah.

He teaches about being appointed over the Tzibur. If he knows one Maseches, he can be appointed where they are learning that Maseches. If he knows everything, he can be appointed Rosh Yeshivah.

Ta'anis 9b: R. Simi bar Ashi regularly learned from Rav Papa, and asked many questions. Once, he saw Rav Papa fall on his face and say 'Heaven save me from embarrassment due to Simi!' Simi accepted to be quiet, and he did not ask any more questions.

Rishonim

Rambam (Hilchos Talmud Torah 4:6): One may not ask the Rav when he enters the Beis Medrash until his mind is settled. A Talmid may not ask when he enters, until he sits and rests. Two may not ask at once. We do not ask the Rav about a different matter, only about the matter being learned, so he will not be embarrassed. A Rav may deceive Talmidim through his questions and actions in front of them to sharpen them and to see whether or not they remember what they learned.

Kesef Mishneh: We learn from Shabbos 3b.

Rambam (7): We ask only b'Inyan (in the matter being learned).

Kesef Mishneh: In Halachah 6, the Rambam taught that when they are learning laws of Shabbos, one may not ask about Yom Tov. Here he teaches that when they are learning about transferring from one Reshus to another on Shabbos, one may not ask about what one may do about a barrel that broke on Shabbos, even though it also pertains to Shabbos.

Rada (in Likutim in Frankel Rambam): In Halachah 6, the Rambam discusses before they started to learn. Then, one may ask about that Maseches or those Halachos. Here he discusses after they began learning. Then, one may ask only about the Halachah being learned. This is not due to embarrassment. Rather, there is no reason to prefer learning the other Halachah to what is being taught.

Chasam Sofer (Shabbos 3b DH Bar): Surely, Rav did not ask about Shabbos when Rebbi was teaching a different matter! We must say that at the time, Rebbi was engaged in Shabbos, just that day he taught about something else. Rav asked not at the time of Beis Medrash, about Shabbos, and even so, R. Chiya was disturbed. Why was he disturbed? A Chacham is qualified to be appointed leader if one can ask him about anything, and he can answer, even if he is not engaged in it now! Also, why did R. Chiya say 'he might have answered incorrectly'? Rav did not ask a difficulty. He asked about a ruling. R. Chiya should have said ' he would have ruled incorrectly'! It seems that had Rav explained his Safek, that perhaps Akirah of his body is like Akirah of his hand in the Mishnah, and he is exempt, or perhaps it is different, then Rebbi would have known from where the Safek came. Since Rebbi knew all of Shas and could answer anywhere, he would have thought about every source to distinguish tem, and he would answer correctly, even if he was engaged in a different Maseches. However, Rav asked Stam 'is Akirah of his body like Akirah of an object', and since Rebbi was engaged in another Maseches, if he were not a great Chacham, he would not remember every contradiction to this reasoning, he would have answered that they are the same. Then, Rav would have asked from the Mishnah, and Rebbi would have given a poor answer to dispel the question, like the Gemara often does. Since Rebbi was a great Chacham, he understood the source of the question and answered properly. Perhaps this is the Kesef Mishneh's intent in Halachah 6. According to what I said, we can answer differently what is the Chidush of Halachah 7 (we ask only what is being learned that day). The Yerushalmi says that Rebbi did not answer Rav immediately, until the next time he learned Shabbos.

Rambam (Hilchos Mechirah 14:14): If a matter of Chachmah was asked, one may not ask Ploni his opinion if Ploni does not know this Chachmah. The same applies to all similar cases.

Magid Mishneh: The Gemara (Bava Metzia 58b) did not mention this, but surely one may learn it from what was taught there.

R. Gershom (Chulin 6a DH v'Im): If you think that your Rebbi cannot answer, do not ask. If you are a Ba'al Nefesh and you want to learn, go to a different Rebbi!

Rashi (Shabbos 114a DH b'Chol): One may be appointed over the Tzibur if people can ask him any Halachah, even 'Kalah', a Beraisa (a minor found after Maseches Avodah Zarah) that most are not familiar with.

Teshuvas Chasam Sofer (7:4): Even though Rebbi was Rosh Yeshiva, and he was greater than all other Chachamim of his generation (Gitin 59a), surely it is a toil when he is asked about a different matter. R. Chiya rebuked Rav that he should not burden Rebbi.

Poskim

Shulchan Aruch (CM 228:4): If a matter of Chachmah was asked, one may not ask Ploni his opinion if Ploni does not know this Chachmah. The same applies to all similar cases.

Shulchan Aruch (YD 246:12): One may not ask the Rav when he enters the Beis Medrash until he sits. A Talmid may not ask when he enters, until he sits and rests. Two may not ask at once. We do not ask the Rav about a different matter, only about the matter being learned, so he will not be embarrassed.

Prishah (30): Two may not ask at once, lest the Rebbi get mixed up.