Evolution of Tefilah - Study of Friday Night Prayer

Siddur of R. Amram Gaon (Goldshmidt edition, p 62)

A question was posed to Rav Natronai: On Friday night, do we say the same final blessings after Shema that we say during the weekday or do we add a special ending, "V'Shomru"? Rav Natronai send back the following answer.

Our Rabbis said that the 3rd Brakho that we say after Shema is not essential, because it is a 5th Brakha. As we learned (Brakhot Mishnah 1:4), in the morning we say two blessing before, "Yozer Hamyorot" and "B'Amo Yisroel", and one blessing after, "G'oal Yisroel". In the evening we say two blessings before, "Maariv" and "V'Ahav", and two after "G'oal Yisroel" and "V'Shomer Amo Yisroel", and nothing more.

And it says in the Brakhot 27b, "As Rav Yehuda said that Shmuel said with regard to the evening prayer. Rabban Gamliel says: It is obligatory. Rabbi Yehoshua says: it is optional. The halakha is in accordance with the statement of the one who said: The evening prayer is obligatory. Rav said: The halakha is in accordance with the statement of the one who said: The evening prayer is optional." According to Shmuel, who says that the halakha is in accordance with the statement of the one who said that the prayer is obligatory, one ends with V'Shomer Amo Yisroel and then right away starts Amidah, but according to Rav, who says that the halakha is in accordance with the statement of the one who said that the prayer is optional, one interrupts, says Kdusha and then says the Amidah.

In all cases where Rav and Shmuel argue, and the Gemara doesn't rule in favor of one or the other, the rule is according to Rav in the issues of ritual matters and according to Shmuel in the issues of contract. Since the final halakha is in accordance with Rav, who says evening prayer is optional, the later Rabbis interrupted after Shomer Amo Yisroel and added statements of praise and song and then also added a brakho in the end. This is done only on weekdays. However, on Shabbat, since there are dangers in the night that may harm Jews, the Jews need to return home before it gets dark and before the lights go out at their home, therefore they only said two brakhot before and two brakhot after, and this is the custom in our Yeshiva and in the houses of the Rabbis in Babylon. This is also what Rabbenu Sar Shalom did, that is that on weekdays, Shabbat and Yom Tov, one only says Shomer Amo Yisroel.

However, in the Yeshiva of our teacher [which Gaon is this?] the custom was that instead of saying Shomer Amo Yisroel, they would end with "Poras Sukat Sholom Aleynu V'al Kol Adat Amo Yisroel V'al Yirushalayim", and they then added Kaddish.

Further, in some prayer houses and some places, they say "Shomer Amo Yisroel Lo'ad" and then they add "V'Shomru Bnei Yisroel" and they end. However, no one adds "Baruch Ato Hashem" after saying "V'Shomru Bnei Yisroel".