מֵאֵימָתַי קוֹרִין אֶת שְׁמַע בְּשַׁחֲרִית. מִשֶּׁיַּכִּיר בֵּין תְּכֵלֶת לְלָבָן. רַבִּי אֱלִיעֶזֶר אוֹמֵר, בֵּין תְּכֵלֶת לְכַרְתִּי. וְגוֹמְרָהּ עַד הָנֵץ הַחַמָּה. רַבִּי יְהוֹשֻׁעַ אוֹמֵר, עַד שָׁלֹשׁ שָׁעוֹת, שֶׁכֵּן דֶּרֶךְ בְּנֵי מְלָכִים לַעֲמֹד בְּשָׁלֹשׁ שָׁעוֹת. הַקּוֹרֵא מִכָּאן וְאֵילָךְ לֹא הִפְסִיד, כְּאָדָם הַקּוֹרֵא בַתּוֹרָה:
Tosefta Berakhot
Rebbi Shimon says, “Sometimes a person may read it (i.e. The Shema) two times in one night, once before dawn and once after dawn. [And if that happens] it comes out that he fulfills his obligation for the day [reading] and for the night [reading].”...
Tosefta Berakhot
From when do we read the Shema in the morning? Acherim (Others) say, “[from the time] when his friend will be four amot away from him and he recognizes him.” The mitzvah [to say it] is with sunrise in order to connect redemption (Geulah) to prayer (Tefillah), and it will come out that he will pray during the day. Rebbi Yehudah said, “One time I was walking behind Rebbi Akiva and Rebbi Elazar Ben Azaryah and the time came to read the Shema. I supposed they gave up on reading the Shema [right away] because they were busy with the needs of the community. I read [the Shema] and [then] learned [some Torah] and after that they began [reading the Shema], and the sun could already be seen above the mountain tops.”