Mishnah 2

מֵאֵימָתַי קוֹרִין אֶת שְׁמַע בְּשַׁחֲרִית? מִשֶּׁיַּכִּיר בֵּין תְּכֵלֶת לְלָבָן. רַבִּי אֱלִיעֶזֶר אוֹמֵר: בֵּין תְּכֵלֶת לְכַרְתִי, וְגוֹמְרָהּ עַד הָנֵץ הַחַמָּה. רַבִּי יְהוֹשֻׁעַ אוֹמֵר: עַד שָׁלֹשׁ שָׁעוֹת, שֶׁכֵּן דֶּרֶךְ בְּנֵי מְלָכִים לַעֲמֹד בְּשָׁלֹשׁ שָׁעוֹת. הַקּוֹרֵא מִכַּאן וָאֵילָךְ – לֹא הִפְסִיד, כְּאָדָם הַקּוֹרֵא בַתּוֹרָה.

From when may we recite the Shema in the morning? From when one can distinguish between Tcheilet and white.Rabbi Eliezer says: [the earliest time for the Shema is when one can distinguish] between tchelet and the color of leek, and one must finish reciting it by sunrise. Rabbi Yehoshua says: until three hours [of the day], since it is customary for princes to rise at the third hour. If one reads [the Shema] later than this, he has not lost out, [but rather is] like one who reads the Torah.