Gemara

Once, Chachamim were alighting to Yerushalayim. When they reached Har ha'Tzofim, they tore their garments.

Mo'ed Katan 20b (Rav Ashi): One tears while standing.

22b (Beraisa): For all Mesim, one tears a Tefach. Even if he is wearing 10 garments it suffices to tear the outermost one. For a parent, one tears all his garments until he bares his heart. Both men and women tear;

R. Shimon ben Elazar says, a woman tears her inner garment, turns it around so the tear is in back), then tears the outer garment (lest she be exposed).

For all Mesim, one may be Sholel (sew back unevenly) after seven days and be Me'acheh (sew back properly) after 30 days. For a parent, one may be Sholel after 30 days, but he may never be Me'acheh. A woman is Sholel immediately for her dignity.

For all Mesim, one tears mibi'Fnim (from the inside), by hand or using a Keli. For a parent one must tear mib'Chutz (from the outside), by hand.

26a: One who sees the Mikdash says "Beis Kodsheinu v'Sif'arteinu Asher Hilelucha Avoseinu Hayah li'Srefas Esh v'Chol Machamadeinu Hayah l'Charbah", and tears.

(Beraisa): One may not be Me'acheh a tear over the Mikdash, but he may be Sholel, hem, Lelaktan (hold the entire tear, and put a needle through it two or three times) or sew it (back unevenly,) like ladders.

Rishonim

Rambam (Hilchos Ta'anis 5:16): One who sees the Mikdash says "Beis Kodsheinu v'Sif'arteinu... Hayah li'Srefas Esh... " and tears.

Rambam (17): Whenever he tears, it is with his hand, while standing. He tears all his garments until he exposes his heart. He may never be Me'acheh, but he may be Sholel, hem, Lelaktan or sew like ladders.

Rebuttal (Ra'avad): The Gemara connotes otherwise. A Tosefta equates these tears to those over a parent only for Ichuy, but not regarding a Keli or baring his heart. Only a Nasi is equated to a parent for tearing outside in front of people.

Magid Mishneh: The Rambam equates tearing over the Mikdash to tearing over a parent for all of these. The Ramban holds like the Ra'avad.

Lechem Mishneh: The Rambam holds that since tearing over the Mikdash is equated to tearing over a parent for Ichuy (26a), it is equated for everything.

Rambam (18): If he came to Yerushalayim and returned, if it is within 30 days he does not tear. If it is after 30 days, he tears again.

Rambam (Hilchos Evel 9:2): One tears for the Mikdash the same way he tears for a parent.

Lechem Mishneh (3): In YD 340, the Tur rules like the Rosh and the Ra'avad, unlike the Rambam. Why does he rule like the Rambam in OC 561? In YD 242, the Beis Yosef answered that the Rambam holds that bearing the heart is included in the Isur of Ichuy. This does not suffice. The Rambam equates other tearings to a parent also for using the hand and tearing all the garments he is wearing. Perhaps the Rambam had a different text.

Rosh (Mo'ed Katan 3:64): One who tears over the Mikdash may never sew it back normally. The Yerushalmi says that if one regularly goes to Yerushalayim, he tears only if it is more than 30 days (from the last time).

Rosh (65): One may be Sholel, hem, Lelaktan or sew like ladders, i.e. after seven days. He may never be Me'acheh/more

Poskim

Shulchan Aruch (OC 561:2): One who sees the Mikdash says "Beis Kodsheinu v'Sif'arteinu... Hayah li'Srefas Esh... " and tears a Tefach.

Beis Yosef (DH u'Mah she'Chosav umi'Heichan): Seeing from farther than Tzofim is not considered seeing.

Magen Avraham (3): Tosfos says that Tzofim is any place from which the Mikdash can be seen. The Tosfos Yom Tov says that Rashi agrees.

Mishnah Berurah (4): Some say that presumably, the first time he sees Yerushalayim, he may not eat meat or drink wine that day.

Mishnah Berurah (5 and Likutei Halachos Zevachim, p.132 in Zivchei Todah): The Rambam and many others say that the Kedushah was permanent (only the Ra'avad disagrees), so one who enters the place of the Mikdash nowadays (that we are all Tamei Mes) is Chayav Kares.

Kaf ha'Chayim (8): One may go close to the openings of the Mikdash and in the Aliyos (upper stories) around the Mikdash.

Mishnah Berurah (6): The Bach says that he bows, tears, cries, mourns and laments. He says Mizmor l'Asaf (Tehilim 79). When he tears, he says Baruch Dayan ha'Emes (without Hash-m's name and Malchus) Ki Kol Mishpatav Emes...

Kaf ha'Chayim (5): One blesses without Shem and Malchus because the Berachah was only for news of a new loss, but not for an old Avelus. After tearing, he says (the end of Tehilim 90, Tefilah l'Moshe) "Samchenu ki'Ymos Anisanu... Yera'eh El Avadecha..." Yehi Ratzon mi'Lfanecha she'Tizkenu b'Rachemecha ha'Rabim Lir'os Binyan Beis ha'Mikdash v'Yerushalayim v'Arei Yisrael v'Yehudah Ken Yehi Ratzon Amen.

Mishnah Berurah (7): If one tore from where can see a little, he need not tear again later when he arrives. L'Chatchilah, one should not tear until he reaches Tzofim.

Shulchan Aruch (4): Whenever he tears, it is with his hand, while standing. He tears all his garments until he exposes his heart.

Magen Avraham (4): Therefore, one tears on the left side.

Kaf ha'Chayim (21): The Beis Yosef brings the Ra'avad, who does not require tearing by hand or exposing the heart.

Question (R. Akiva Eiger): In Sa'if 2, we said that he tears only a Tefach (from the top of the garment). This is not enough to expose the heart!

Answer (Kaf ha'Chayim 22): He exposes the flesh even with the heart.

Shulchan Aruch (ibid): He may never sew it back normally, but he may be Sholel, hem and Lelaktan and sew like ladders.

Magen Avraham (5): Ichuy is sewing on the inside, and afterwards outside. Rashi says that Ichuy is straight sewing.

Kaf ha'Chayim (23): Some forbid even one straight stitch.

Mishnah Berurah (15): He may be Sholel the next day.

Shulchan Aruch (5): If he came to Yerushalayim and returned, if it is within 30 days he does not tear. If it is after 30 days, he tears again.

Magen Avraham (6): One who was born in Yerushalayim need not tear even after he matures. When he was a minor, he was exempt. After he matures he is exempt, for he saw it within 30 days.

Kaf ha'Chayim (26): The Radvaz says that if a resident of Yerushalayim left for 30 days, he tears when he returns. The custom is not like this.

Halichos Shlomo 16(17): No Poskim exempt from tearing on days (other than Shabbos and Mo'adim) when we do not say Tachanun, but people are lenient about this.

Teshuvos v'Hanhagos (5:165,166): Perhaps the Chiyuv to tear is only if one sees (from a high place) the ground of the Mikdash, or at least the mosque. Perhaps residents of Eretz Yisrael may be lenient if they do not see even the mosque. One who is stringent to tear will be blessed The ideal Mitzvah to tear is from a place where see the ground, and amidst great anguish. One should not rely on a Kinyan in which he 'gives' his clothes to another to exempt himself, for this is a ruse. Perhaps one who borrows another's clothing is exempt. There is a greater Chiyuv to tear the first time one sees the Mikdash. If one takes off his jacket, it suffices to tear his shirt.

Halichos Shlomo 6(15): It is proper to see the ground of the Mikdash during festivals to commemorate the Mitzvah of entering the Mikdash each Regel.

Halichos Shlomo (6:41): people were never concerned about putting one's hand between rocks of the Kosel. (Ha'Gaon R. Y.Y. Fisher forbids coming closer than a foot from the Kosel, for this is directly above recessed lower layers of stones, and it has Kedushas Har ha'Bayis - Teshuvos v'Hanhagos (5:164)

Igros Moshe (YD 4:63 DH v'Gam): One may not remove pebbles or dirt from between the rocks.