Gemara

6a (Beraisa - Aba Binyamin): One's prayer is heard only if he prays in a Beis ha'Keneses:

"Lishmo'a El ha'Rinah v'El ha'Tefilah" -- the place where we sing (Hash-m's praises) is the proper place for Tefilah.

7b - R. Yitzchak (to Rav Nachman): Why didn't you pray in the Beis ha'Keneses?

Rav Nachman: I was too weak.

R. Yitzchak: You should have gathered ten men to pray with you!

Rav Nachman: That is an exertion.

R. Yitzchak: You should have asked a messenger to tell you when the congregation prays, so that you could pray at the same time!

Question (Rav Nachman): Why is this important?

Answer (R. Yitzchak citing R. Yochanan): "Va'Ani Sefilasi Lecha Hash-m Es Ratzon" -- the time when Hash-m desires prayer is when the congregation prays.

Support (Beraisa - R. Nasan): Hash-m does not despise the prayer of a multitude -- "Hen Kel Kabir...";

It also says "Podah v'Shalom Nafshi..." Hash-m says 'if one engages in Torah and Chesed and prays with the congregation, I consider it as if he redeemed Me and My children from the nations.'

Avodah Zarah 4b (Rav Yosef): An individual should not pray Musaf during the first three hours of the day on Rosh Hashanah. Since Hash-m judges then, perhaps He will scrutinize his deeds, and his prayer will be rejected.

The congregation may pray then. Since it has many merits, its prayer is heard.

Question: Likewise, an individual should not pray Shacharis at this time!

Answer: Since the congregation prays at this time, his prayer will not be rejected.

Rishonim

Rif and Rosh (4a and 1:7): Aba Binyamin learns from "Lishmo'a El ha'Rinah v'El ha'Tefilah" that one's prayer is heard only if he prays in a Beis ha'Keneses." R. Yochanan learned this from "va'Ani Sefilasi Lecha Hash-m Es Ratzon" -- the time that Hash-m desires prayer is when the congregation prays.

R. Yonah (4a): The Ge'onim say that when a person cannot go to the Beis ha'Keneses, he should pray when the Tzibur prays. Even at a time that the Tzibur does not pray, one should pray in the Beis ha'Keneses, for it is a fixed place special for Tefilas ha'Tzibur. 'One's Tefilah is heard only in the Beis ha'Keneses' does not refer to one for whom Toraso Umnaso (he has no other occupation). Such a person may pray in his Beis Midrash.

Rambam (Hilchos Tefilah 8:1): Tefilah b'Tzibur is always heard, even if there are sinners among them. Hash-m does not despise the Tefilah of the Rabim. Therefore one must join with the Tzibur and not pray alone if he can pray with the Tzibur. One should be early and late in the Beis ha'Keneses, for Tefilah is always heard only in the Beis ha'Keneses.

Kesef Mishneh: We learn from 8a that even if one does not go to the Beis ha'Keneses, he should pray when the Tzibur prays. This is the Rambam's intent 'one should not pray alone.'

Tosfos (Avodah Zarah 4b DH Kivan): Our Gemara connotes that a Tefilah at the same time that the Tzibur prays is not rejected, even if it is not in the same place. R. Tam explains that it is not rejected, but it is heard only if it is where the Tzibur prays, like it says in Berachos that one's Tefilah is heard only with the Tzibur.

Poskim

Shulchan Aruch (OC 90:9): One should strive to pray in the Beis ha'Keneses with the Tzibur.

Beis Yosef (DH Lo): R. Yochanan taught that one's prayer is heard (only in a Beis ha'Keneses). The Tur explains that this means with the Tzibur. He connotes that if one does not pray with the Tzibur, there is no difference between praying in the Beis ha'Keneses or in the house. However, R. Yonah brings from Ge'onim that it is better to pray in the Beis ha'Keneses.

Kaf ha'Chayim (51): The Beis Yosef brought this from R. Yonah, and not from the Gemara itself, for one could explain the Gemara like the Tur.

Magen Avraham (15): Even if he could pray in his house with a Minyan, it is better in the Beis ha'Keneses due to "b'Rav Am Hadras Melech" (it is better to do Mitzvos amidst many people).

Mishnah Berurah (28): If there are two Batei Kenesiyos and one of them has many people, it is a greater Mitzvah to pray there. The Acharonim say that if the Beis ha'Keneses with more people is noisy, and one does not hear the Tefilah and not Keri'as ha'Torah, it is better to pray in one's house with 10.

Kaf ha'Chayim (54): Similarly, if in the big Tzibur, due to bad colleagues one will stumble in idle words, scoffery... it is better to pray with 10 in his house.

Mishnah Berurah (28): Tefilah b'Tzibur in one's house is better than praying alone in the Beis ha'Keneses, even if he would be able to hear Kaddish and Borchu. The primary Tefilah b'Tzibur is Shemoneh Esre, i.e. 10 adult men pray together. Commoners think that the primary attribute of praying with 10 is to hear Kaddish, Kedushah and Borchu, so they are particular only that there be 10 in the Beis ha'Keneses. This is wrong. One must hurry to the Beis ha'Keneses so he will pray Shemoneh Esreh b'Tzibur. If on Shabbos or Yom Tov one prays early in a small Minyan because the big Minyan prays late, and they pray also Musaf in the small Minyan, and there is time to go to the big Minyan for Musaf, it is good to go to pray Musaf with the big Minyan, due to "b'Rav Am" (Yad Eliyahu 42). This is only if his fixed place to pray is in the Beis ha'Keneses. If not, it is better to pray the entire Tefilah in one place. The Radvaz taught that if a prisoner is permitted to pray one day with a Minyan, he should not delay the Mitzvah to wait for Yom Kipur or Purim. (Rather, he prays with a Minyan the first day possible.)

Kaf ha'Chayim (56): If it is hard for an old person to leave his house when it is cold in winter, letter of the law he may pray alone in his house.

Kaf ha'Chayim (57): If the only possibility to pray with a Minyan requires traveling alone for half an hour to a village before dawn, one may not endanger himself in order to do so.

Kaf ha'Chayim (58): If one was about to pray Musaf and they asked him to circumcise, he should not miss Tefilah b'Tzibur for this, unless there is concern that if so, the Bris will not be done that day. Surely one should not miss hearing the Megilah b'Tzibur in order to circumcise, since the Mitzvah is on the father, and not on him.

Kaf ha'Chayim (53): Sefer Chasidim says not to pray near a Rasha, for this will cause him to have bad thoughts, and the Shechinah distances from him. One should be careful about this when possible.

Shulchan Aruch (ibid.): If due to Ones he cannot go to the Beis ha'Keneses, he should coordinate to pray when the Tzibur prays.

Magen Avraham (16): Even if one is weak, this is called Ones. Had Rav Nachman been sick, R. Yitzchak would not have asked him why he does not pray in the Beis ha'Keneses. Rather, he was merely weak.

Magen Avraham (17): He prays at the same time as the Tzibur only if it is difficult to gather a Minyan. Smag connotes that Rav Nachman did not want to burden the others to come to him. Avodah Zarah 4a connotes that if one prays Musaf when the Tzibur prays Shacharis, this is not considered when the Tzibur prays. Tosfos says that Tefilah at the same time that the Tzibur prays is not rejected, but it is heard only with the Tzibur in the Beis ha'Keneses. However, perhaps Tosfos said so only at a time of anger (like the Gemara discussed).

Mishnah Berurah (29): If there is monetary Ones, that if he tries to pray with a Minyan he will lose money, he may pray alone in his house or in the Beis ha'Keneses. One may not refrain from Tefilah b'Tzibur if this will merely prevent him from profiting. There is a difference between a loss and not profiting. Eliyahu Rabah says that we should fine people who refrain from coming to the Beis ha'Keneses because they are learning or earning money. They should fine rich people more. Even if a Chacham is learning, there is suspicion and Chilul Hash-m (if he does not come to the Beis ha'Keneses).

Kaf ha'Chayim (63): Praying silently during Chazaras ha'Shatz is called praying at the same time as the Tzibur.

Kaf ha'Chayim (64): Praying (Shemoneh Esre) at the same time as the Tzibur means when they are praying Shemoneh Esre. Even so, it is better to pray when they are saying other parts of the Tefilah than when they are not praying at all.

Maharil (Hilchos Eruvei Chatzeros 7, cited in Kaf ha'Chayim 66): One may make Eruvei Techumim only for a Mitzvah. The only Poskim who permit it in order to pray with a Minyan are Semak and the Agudah. Why is this a smaller need than one who wants to go to the Beis ha'Mishteh? Praying with a Minyan is not such a big Mitzvah, for one can be Mekaven (perhaps this means to have intent, or to pray when the Tzibur prays - PF) Tefilah in his house. We do not find that Chachamim obligated to pray with 10.

Yad Eliyahu (6, b'Sof, cited in Kaf ha'Chayim 67): If one was invited to a Seudas Mitzvah and if he goes he will not be able to pray b'Tzibur, he may go, for one who is engaged in a Mitzvah is exempt from other Mitzvos. However, if he would be able to pray later b'Tzibur, just he would not do so due to matters of honor, he may not put himself in such a situation. One who refrains in such a case is not excommunicated to Shamayim.

Mishnah Berurah (30): Avodah Zarah 4b connotes that if one prays Musaf when the Tzibur prays Shacharis, this is called Tefilah b'Tzibur only if he is with them in the Beis ha'Keneses.

Rema: The same applies to people who live in settlements and there is no Minyan. In any case they should pray Shacharis and Ma'ariv at the time the Tzibur prays.

Magen Avraham (18): In summer, when the Tzibur delays praying Shacharis, one who lives in a settlement may not even learn before praying (89:6), therefore it is better to pray immediately at sunrise (lest he be idle from Torah).

Magen Avraham (19): In Ashkenaz, where they pray Ma'ariv early (before Tzeis ha'Kochavim), if an individual (who cannot pray in a Minyan, e.g. he) is in a village or on the road, he prays at Tzeis ha'Kochavim.

Shulchan Aruch: Similarly, if due to Ones one did not pray when the Tzibur prays and he prays alone, even so he should pray in the Beis ha'Keneses.

Kaf ha'Chayim (70): If he will be distracted due to fear of being alone, he should pray in his house.

Mishnah Berurah (33): If there is concerned for Chilul Hash-m if a Chacham will come to the Beis ha'Keneses to pray after the Tefilah (of the Tzibur), surely he should pray in his house.