Maseches Ta'anis, Chapter 2, Mishnah 5. This mishnah relates an incident that took place on one of the fast days, and from it we learn how the additional prayers of a fast day were formulated and how the congregation responded to the berachos.
The wording of the mishnah and the incident it records:
"Ma'aseh bimei Rabbi Chalafta v'Rabbi Chananya ben Teradyon" - the incident took place in the days of these two sages.
"She'avar echad lifnei ha'teivah" - a man served as the chazzan for the congregation.
"V'gamar es ha'berachah kulah" - he completed the berachah in its entirety, meaning the first berachah with the closing "Go'el Yisrael."
"V'lo anu acharav amen":
The congregation did not answer amen after him. What did they say instead? The Gemara explains that they said "Baruch shem kevod malchuso l'olam va'ed" in place of amen, and this was the practice they were accustomed to in the Beis HaMikdash.
The order of the blasts and the declarations:
Afterward the attendant of the synagogue, the chazzan ha'knesses, announced: "Tik'u ha'kohanim tik'u" - blow, kohanim, blow. And after the blast they said: "Mi she'anah es Avraham Avinu b'Har HaMoriah hu ya'aneh eschem v'yishma b'kol tza'akaschem hayom hazeh" - He who answered Avraham Avinu on Har HaMoriah, He will answer you and hear the sound of your cry on this day. This is the declaration we saw in the previous mishnah, and it is said after the first berachah.
After the next berachah, which is the berachah of Zichronos, they said: "Hari'u bnei Aharon hari'u" - sound the teruah, sons of Aharon, sound the teruah. Here the mishnah changed its wording in two details:
Instead of calling them "kohanim," it says "bnei Aharon."
Instead of "tik'u," it says "hari'u" - the matter of the teruah blast.
And afterward they would say: "Mi she'anah es avoseinu al Yam Suf hu ya'aneh eschem v'yishma b'kol tza'akaschem hayom hazeh" - He who answered our forefathers at the Yam Suf, He will answer you and hear the sound of your cry on this day. And so they did for each and every berachah, that the same procedure was carried out with the remaining berachos as well.
"U'ksheba davar eitzel chachamim":
When this incident was reported to the Sages - and the reference is particularly to the fact that instead of answering amen to the berachah they answered "Baruch shem" - the Sages said: "Lo hayinu nohagim kein ela b'sha'ar ha'mizrach u'v'Har HaBayis" - we followed this practice only at the eastern gate and on Har HaBayis. In other words, this practice of refraining from saying amen and saying "Baruch shem" in its place applied only in the Mikdash and nowhere else.
In summary: In this mishnah we learned of an incident that took place in the days of Rabbi Chalafta and Rabbi Chananya ben Teradyon: the chazzan completed the entire berachah, and the congregation answered after him "Baruch shem kevod malchuso l'olam va'ed" instead of amen. We examined the attendant's announcements - "Tik'u ha'kohanim tik'u" after the first berachah and "Hari'u bnei Aharon hari'u" after the berachah of Zichronos - and the declarations that accompanied them at each and every berachah. And finally we learned the ruling of the Sages, that this practice of "Baruch shem" in place of amen was unique to the eastern gate and Har HaBayis alone.