Mishnayos Brachos Perek 1 Mishnah 4

From the laws of the recitation of Shema itself, the mishna proceeds to discuss the blessings recited in conjunction with Shema. Here, the order is established: In the morning when reciting Shema, one recites two blessings beforehand, the first on the radiant lights and the second the blessing on the love of Torah, and one thereafter, which begins with: True and Firm [emet veyatziv]. And in the evening one recites two blessings beforehand, on the radiant lights and on the love of God, and two thereafter, the blessing of redemption: True and Faithful [emet ve’emuna], and the blessing: Help us lie down. With regard to the blessing: True and Faithful, whether one recites it in its long formula and whether one recites it in its short formula, he fulfills his obligation (Tosafot). However, the general principle is: Where the Sages said to recite a long blessing, one may not shorten it, and so too, wherever they said to recite a short blessing, one may not lengthen it. Where the Sages said that a blessing must conclude with a second blessing at the end, he may not fail to conclude with that blessing. Similarly, if the Sages said that a blessing must not conclude with a second blessing, one may not conclude with a blessing.

בַּשַּׁחַר מְבָרֵךְ שְׁתַּיִם לְפָנֶיהָ וְאַחַת לְאַחֲרֶיהָ, וּבָעֶרֶב שְׁתַּיִם לְפָנֶיהָ וּשְׁתַּיִם לְאַחֲרֶיהָ. אַחַת אֲרֻכָּה וְאַחַת קְצָרָה. מָקוֹם שֶׁאָמְרוּ לְהַאֲרִיךְ, אֵינוֹ רַשַּׁאי לְקַצֵּר. לְקַצֵּר, אֵינוֹ רַשַּׁאי לְהַאֲרִיךְ. לַחְתֹּם, אֵינוֹ רַשַּׁאי שֶׁלֹּא לַחְתֹּם. וְשֶׁלֹּא לַחְתֹּם, אֵינוֹ רַשַּׁאי לַחְתֹּם: