אורח חיים מ״טOrach Chayim 49

Se’if 1א׳

אַף עַל גַּב דְּקַיְמָא לָן דְּבָרִים שֶׁבִּכְתָב אִי אַתָּה רַשַּׁאי לְאָמְרָם עַל פֶּה, כָּל דָּבָר שֶׁרָגִיל וְשָׁגוּר בְּפִי הַכֹּל, כְּגוֹן קְרִיאַת שְׁמַע וּבִרְכַּת כֹּהֲנִים וּפָרָשַׁת הַתָּמִיד וְכַיּוֹצֵא בָּהֶן, מֻתָּר:

Even though we have established that things [from the Torah] which were written, you have no permission to recite them by heart, any thing that is routine and fluent in everyone's mouth, for example, the recitation of the Sh'ma and the Priestly Blessing and the section of the daily offering, and similar things to them is permitted.