Gemara

When R. Yochanan died, R. Ami mourned Shivah and Sheloshim.

(Abaye): This is not the Halachah;

(R. Chiya bar Aba citing R. Yochanan): One mourns only one day over his Rebbi, even if he learned most of his Chachmah from him.

(Beraisa): Ichuy (sewing back) is forbidden for the following tears...one who tore for the Rebbi from whom he learned Chachmah...

Question: What is the source of this?

Answer: When Elisha saw Eliyahu (go up in the wind) he screamed "Avi Avi Rechev Yisrael u'Farashav...vaYachazek bi'Vgadav va'Yikra'em li'Shnayim Kera'im";

"Avi Avi" alludes to (tearing for one's) father and mother; "Rechev Yisrael u'Farashav" refers to the Rebbi who taught to him Torah.

Question: How do we learn this from the verse?

Answer: Rav Yosef said that the Targum of this verse is 'Rebbi Rebbi who helps Yisrael with his Tefilah more than chariots and soldiers on horses.'

Question: What is the source to forbid Ichuy?

Answer - Question: "Va'Yikra'em" implies that he tore his garments in two. Why do we need "li'Shnayim Kera'im"?

Answer: This teaches that they will always be two.

Question (Reish Lakish): Eliyahu did not die!

Answer (R. Yochanan): Since Elisha did not see him again, it is as if he died.

22b - Beraisa: For all Mesim (except for a parent) one tears a Tefach. Even if he is wearing 10 garments it suffices to tear the outermost one. He may be Sholel (sew it with wide stitches, or tie the threads on the two sides of the tear) after seven days and be Me'acheh (sew it back properly) after 30 days. For a parent, one tears all his garments, until he bares his heart. He may be Sholel after 30 days, but he may never be Me'acheh.

For all Mesim, one may tear by hand or using a Keli; he tears (from the) inside. For a parent one must tear by hand, parent one tears (from the) outside.

(Rav Chisda): The same applies to tearing for the Nasi.

Question (Beraisa): (A Beraisa (26a) teaches about tears for which Ichuy may never be done, e.g. one's Rebbi, the Av Beis Din and the Nasi.) All of these are like a parent only regarding Ichuy.

Suggestion: This refers even to the Nasi!

Answer: No, it refers only to the others.

Rishonim

Rif and Rosh (3:64): Ichuy is forbidden for a tear for the Rebbi who taught to him Torah.

Nimukei Yosef (15a DH Gemara): We tear and cry over the death of any Chacham, but Aveilus is only for one's Rebbi.

Rambam (Hilchos Evel 9:2): Just like one tears for a parent, he tears for the Rebbi who taught to him Torah, for the Nasi...

Rambam (3): All of these tears are until he bares his heart. He never does Ichuy Alexandri. It is permitted Lemolelan, Lesholelan, Leloktan, or to make like ladders.

Question (Ramban, brought in Kesef Mishneh): Except for Ichuy, only a Nasi has the stringencies of parents (22b)! We learn that Kri'ah must be a Tefach from David. He tore only a Tefach over the Nasi and Av Beis Din (Sha'ul) and bad tidings! The Yerushalmi says that baring the heart is for a parent, to symbolize the loss of the Mitzvah of honoring parents.

Answer #1 (Kesef Mishneh): The Rambam understands that baring the heart is included in the Isur Ichuy, which is learned from Elisha. Had he torn only a Tefach, it would not have been evident that they are Shnayim Kera'im. It is reasonable to bare the heart for a Nasi, Rebbi or Av Beis Din, for there also he loses the Mitzvah of honoring them. The verse about David is an Asmachta to teach that one tears a Tefach for other Mesim.

Questions (Lechem Mishneh): The Kesef Mishneh's answer is not sufficient. The Rambam (Ta'anis 5:17) says that any Kri'ah for which Ichuy is forbidden is just like for a parent, also regarding tearing by hand and tearing all one's garments! Here, the Tur says that a Nasi is like a parent only regarding baring the shoulder and tearing outside. This is like the Ramban. Regarding tearing over the Mikdash, the Tur (OC 561) rules like the Rambam!

Answer #2 (Lechem Mishneh Talmud Torah 5:9): The Beraisa says 'all of these are like a parent only regarding Ichuy'. This means that the permanent Isur of Ichuy is the only stringency particular to the tears listed in the Beraisa. All tears listed in the Beraisa also require baring the heart, but this stringency applies even where Ichuy is permitted, e.g. for a Chacham who was not one's Rebbi (Halachah 11).

Rambam (12): If one tore for a Chacham, he may be Sholel once he turns away from the bier. It seems that he may sew it back normally the next day. Even for one's Rebbi one mourns only one day, the death of death or the day he heard.

Kesef Mishneh: The Ramban says that for one's Rebbi one removes his shoes and observes all laws of Aveilus for part of a day.

Hagahos Maimoniyos (6): He must even change his place in the Beis ha'Keneses.

Radvaz: The Rambam omits the law of a Rebbi who was not his primary Rebbi. It seems that he holds that this is like a Chacham who died.

Rambam (Hilchos Talmud Torah 5:9): When one's Rebbi dies, he tears all his garments until he bares his heart. He may never Me'acheh. This refers to his primary Rebbi who taught him most of his Chachmah. If not, he is a Talmid Chaver, and tears for him like for other Mesim for which one is Mis'avel. This is even if he learned from him only one thing.

Rosh (3:43): One's Rebbi who taught to him Chachmah is like a parent only regarding Ichuy. He observes all Isurei Aveilus for part of a day.

Beis Yosef (YD 242 DH u'Mah she'Chosav Rabeinu): The Rosh requires part of a day, like for a Shemu'ah Rechokah.

Poskim

Shulchan Aruch (YD 242:25): When one's Rebbi dies he tears all his garments, until he bares his heart. Some say that he tears only a Tefach. Ichuy is never permitted. He mourns by removing his shoes and observing all laws of Aveilus for part of the day of death or the day he heard.

Shulchan Aruch (26): One tears for his Rebbi even for a Shemu'ah Rechokah, just like for one's father.

Source (Beis Yosef DH Kosav ha'Ramban): If one tears for his father, who brought him to this world, all the more so he tears for his Rebbi, who brings him to the world to come! The Beraisa says that one's Rebbi is like a parent only regarding Ichuy, among matters mentioned in the Beraisa. It did not mention Shemu'ah Rechokah.

Shulchan Aruch (27): If one's Rebbi is dead in front of him, he does not eat meat and wine, like one who relative is in front of him.

Source: Ramban (brought in Beis Yosef DH v'Chosav Od), based on the Yerushalmi.

Shulchan Aruch (30): These laws apply to one's primary Rebbi from whom he learned most of his Chachmah, be it written Torah, Mishnah or Gemara.

Rema: Nowadays, the primary Rebbi is not the one who taught him Pilpul and distinctions, rather, one who taught Pesak Halachah, investigation and guided him to what is true and straight.

Source: Maharik, brought in Beis Yosef (DH v'Chosav Maharik).

Shulchan Aruch: If he did not learn most of his Chachmah from him, he tears from him like for any relative for which there is Aveilus. This is even if he learned from him one matter, be it big or small.

Shulchan Aruch (340:8): For the Rebbi from whom he learned most of his Chachmah, be it written Torah, Mishnah or Gemara, one tears until he bares his heart. Some say that he tears only a Tefach. He is never Me'acheh. This is even for a Shemu'ah Rechokah. If he did not learn most of his Chachmah from him, he tears like for other Mesim for which one is Mis'avel. This is even if he learned from him only one thing, big or small.

Rema: Some say that one is not Me'acheh even for a non-primary Rebbi. If Chachamim learn together and answer questions for each other, some say that each is like a non-primary Rebbi for each other, and some say that each is like a primary Rebbi for each other. Some say that one must tear only for the Rebbi from whom he learned most of his Chachmah. It is a stringency for colleagues who learn together to tear for each other, or to tear for one from whom he learned one matter. This stringency applies only where it is the custom; we do not tell people to be stringent. This is why people are lenient in our lands.

Shulchan Aruch (374:10): Aninus applies for one's Rebbi, but he blesses, and makes a Zimun. He removes his shoes and observes all laws of Aveilus for part of a day.

Source (Beis Yosef DH v'Al): The Ramban says that the Yerushalmi forbids meat and wine while one's Rebbi is awaiting burial. Perhaps the Yerushalmi discusses only meat and wine, therefore we cannot exempt from Mitzvos without a proof.

Rema: The Beis Yosef says that he must change his place in the Beis ha'Keneses. Part of a day suffices.