Reference for Berakhot 2:3
הַקּוֹרֵא אֶת שְׁמַע וְלֹא הִשְׁמִיעַ לְאָזְנוֹ, יָצָא. רַבִּי יוֹסֵי אוֹמֵר, לֹא יָצָא. קָרָא וְלֹא דִקְדֵּק בְּאוֹתִיּוֹתֶיהָ, רַבִּי יוֹסֵי אוֹמֵר יָצָא, רַבִּי יְהוּדָה אוֹמֵר לֹא יָצָא. הַקּוֹרֵא לְמַפְרֵעַ, לֹא יָצָא. קָרָא וְטָעָה, יַחֲזֹר לְמָקוֹם שֶׁטָּעָה:
One who recites the Shema without causing himself to hear it fulfills the obligation. R. Yossi says: He does not fulfill the obligation. [For it is written (Deuteronomy 6:4): "Hear" — Let your ear hear what your mouth utters. And the first tanna holds: "Hear" — in any language that you are accustomed to hear. And the halachah is according to the first tanna.] If he recited it without being precise with its letters [to enunciate them clearly, in an instance of two words where the second word begins with the same letter with which the first letter ends, as in "al levavcha," "esev besadecha," "va'avadetem meherah." If he does not leave space between them to separate them, it sounds as if he is pronouncing two letters as one.] — R. Yossi says: He has fulfilled his obligation. [And the halachah is according to R. Yossi. However, ab initio, he must enunciate the letters. Likewise, he must take care not to rest the mobile sheva and not to move the quiescent, and not to weaken (by pronouncing without a dagesh) a strong form and not to strengthen a weak one. And he must accentuate the zayin of "tizkeru," so that it does not sound like "tiskeru," that is, "so that you amass reward." For it is not fitting to serve the Master for the sake of reward.] R. Yehudah says: He has not fulfilled his obligation. If one recites it in inverted order [If he recites the third verse before the second, the second before the first, and the like], he has not fulfilled his obligation [it being written (Deuteronomy 6:6): "and these words shall be" — they shall remain in their original form, i.e., as they are ordered in the Torah. However, if he advances the section, reciting vayomer before vehaya im shamoa, and vehaya im shamoa before Shema, it would seem that this is not considered "inverted," and he fulfills his obligation; for they are not thus arranged, one after the other, in the Torah.] If he recited it and erred, he returns to the point of the error. [If he erred between one section and another, not knowing with which section he left off and to the beginning of which section he should return, he returns to the first verse, vehaya im shamoa. (Rambam says: Veahavta eth Hashem.) And if he stopped in the middle of a section, knowing which section, but not knowing where in that section he left off, he returns to the beginning of that section. If he recited "uchethavtam," but did not know whether it were that of Shema or that of vehaya im shamoa, he returns to the "uchethavtam" of Shema. And if he were in doubt after he began leman yirbu, he does not return, for he can rely on "the habit of his tongue."]