Mishnah 4

בַּשַּׁחַר מְבָרֵךְ שְׁתַּיִם לְפָנֶיהָ וְאַחַת לְאַחֲרֶיהָ, וּבָעֶרֶב שְׁתַּיִם לְפָנֶיהָ וּשְׁתַּיִם לְאַחֲרֶיהָ, אַחַת אֲרוּכָה וְאַחַת קְצָרָה. מָקוֹם שֶׁאָמְרוּ לְהַאֲרִיךְ – אֵינוֹ רַשָּׁאי לְקַצֵּר. לְקַצֵּר – אֵינוֹ רַשָּׁאי לְהַאֲרִיךְ. לַחְתֹּם – אֵינוֹ רַשָּׁאי שֶׁלֹּא לַחְתֹּם. וְשֶׁלֹּא לַחְתֹּם – אֵינוֹ רַשָּׁאי לַחְתֹּם.

In the morning one says two blessings before it [the Shema] and one after it, and in the evening he says two blessings before it and two after it, one long [blessing] and one short [blessing]. Where they said to make the blessing long – he may not shorten it. [Where they said] to make the blessing short – he may not lengthen it. [Where they said] to conclude it – he is not permitted to omit the conclusion. [Where they said] not to conclude it – he is not permitted to conclude.