Berakhot 27B

לא כנגד רבו, ולא אחורי רבו.

directly next to his rabbi, presumptuously indicating that he is his rabbi’s equal, and behind his rabbi as it creates the impression that he is bowing to him (Tosafot)?

ותניא, רבי אליעזר אומר: המתפלל אחורי רבו, והנותן שלום לרבו, והמחזיר שלום לרבו, והחולק על ישיבתו של רבו, והאומר דבר שלא שמע מפי רבו ⁠— גורם לשכינה שתסתלק מישראל!

And it was taught in a baraita, in a more extreme manner, as R' Eliezer says: One who prays behind his rabbi and one who greets his rabbi without waiting for his rabbi to greet him first, one who returns his rabbi’s greeting without saying: Greetings to you, rabbi, one who rivals his rabbi’s yeshiva, i.e., establishes a yeshiva of his own and teaches during his rabbi’s lifetime without his consent (Rambam), and one who says something in the name of his rabbi which he did not hear directly from his rabbi, causes the Shekhina to withdraw from Israel.

שאני רבי ירמיה בר אבא דתלמיד חבר הוה. והיינו דקאמר ליה רבי ירמיה בר אבא לרב: מי בדלת? אמר ליה: אין, בדילנא, ולא אמר ״מי בדיל מר״?

With regard to R' Yirmeya’s conduct, the Talmud explains that R' Yirmeya bar Abba is different, as he was not a mere student of Rav. Rather, he was a disciple-colleague and was, therefore, permitted to act that way. And that is why on one occasion, when Rav prayed the Shabbat prayer early, R' Yirmeya bar Abba asked him: Did you distance yourself from labor and accept the sanctity of Shabbat? Rav said to him: Yes, I distanced myself. And R' Yirmeya did not say to him: Did the Master distance himself, as would have been appropriate had he merely been Rav’s student.

ומי בדיל? והאמר רבי אבין: פעם אחת התפלל רבי של שבת בערב שבת, ונכנס למרחץ, ויצא ושנה לן פרקין, ועדיין לא חשכה! אמר רבא: ההוא דנכנס להזיע, וקודם גזירה הוה.

Although Rav replied that he distanced himself from labor, did he indeed need to distance himself from labor? Didn’t R' Avin say: Once R' Yehuda HaNasi prayed the Shabbat prayer on Friday before nightfall. He then entered the bathhouse and emerged and taught us our chapters that we had learned, and it was not yet dark. Rava said: That is a case where he had entered the bathhouse to perspire, and it was before the rabbis issued a decree prohibiting perspiring in a bathhouse on Shabbat.