Talmud for Berakhot 1:1
מֵאֵימָתַי קוֹרִין אֶת שְׁמַע בְּעַרְבִית. מִשָּׁעָה שֶׁהַכֹּהֲנִים נִכְנָסִים לֶאֱכֹל בִּתְרוּמָתָן, עַד סוֹף הָאַשְׁמוּרָה הָרִאשׁוֹנָה, דִּבְרֵי רַבִּי אֱלִיעֶזֶר. וַחֲכָמִים אוֹמְרִים, עַד חֲצוֹת. רַבָּן גַּמְלִיאֵל אוֹמֵר, עַד שֶׁיַּעֲלֶה עַמּוּד הַשָּׁחַר. מַעֲשֶׂה שֶׁבָּאוּ בָנָיו מִבֵּית הַמִּשְׁתֶּה, אָמְרוּ לוֹ, לֹא קָרִינוּ אֶת שְׁמַע. אָמַר לָהֶם, אִם לֹא עָלָה עַמּוּד הַשַּׁחַר, חַיָּבִין אַתֶּם לִקְרוֹת. וְלֹא זוֹ בִּלְבַד, אֶלָּא כָּל מַה שֶּׁאָמְרוּ חֲכָמִים עַד חֲצוֹת, מִצְוָתָן עַד שֶׁיַּעֲלֶה עַמּוּד הַשָּׁחַר. הֶקְטֵר חֲלָבִים וְאֵבָרִים, מִצְוָתָן עַד שֶׁיַּעֲלֶה עַמּוּד הַשָּׁחַר. וְכָל הַנֶּאֱכָלִים לְיוֹם אֶחָד, מִצְוָתָן עַד שֶׁיַּעֲלֶה עַמּוּד הַשָּׁחַר. אִם כֵּן, לָמָּה אָמְרוּ חֲכָמִים עַד חֲצוֹת, כְּדֵי לְהַרְחִיק אֶת הָאָדָם מִן הָעֲבֵרָה:
From which time may the Shema be recited in the evenings? From the time that the Cohanim have gone in to eat their terumah [Cohanim who became unclean and who immersed may not eat terumah until "their sun has set"; that is, until the stars have appeared. The reason "From the time the stars have appeared" is not stated is that we might thereby be apprised of an incidental learning, viz., if the Cohanim become defiled with the type of uncleanliness where their cleansing entails an offering (such as zav or metzora), the want of this expiatory offering does not prevent them from eating terumah, it being written (Leviticus 22:7): "And when the sun has set vetaher ('and the day has ended'), he may eat of the holy things" — the setting of the sun is a prerequisite for his eating terumah, but his expiatory offering is not], until the end of the first watch. [the first third of the night, the night being divided into three watches. From that point on, it is not considered the time of the recital of the Shema of reclining, and it does not satisfy (Deuteronomy 6:7): "…when you lie down." And before the stars come out, too, it is daytime, and not the time of lying down. And those who are beforehand and recite the evening Shema while it is still day, rely in this on R. Yehudah, who says (26a) that the Minchah prayer may be recited until midway through the afternoon, an hour and a quarter before the night. And it is ruled that one may follow R. Yehudah in this regard — that immediately upon the expiration of the time for the Minchah prayer, the time for the recital of the evening Shema begins.] These are the words of R. Eliezer. And the sages say: Until midnight. R. Gamliel says: Until the appearance of the morning star. [For the entire night is considered the time of lying down. And the halachah is in accordance with R. Gamliel, the sages, too, agreeing with him, having said "Until midnight" only to keep one far from transgression. However, ab initio, when the time of the recital of Shema of the Mishnah arrives —- that is, when the stars appear — one is forbidden to repast, and, it goes without saying, to sleep, until he recites the Shema and prays.] It once happened that his sons came late from a feast [The sons of R. Gamliel heard that the sages had said: "Until midnight," and this is what they said to him: Do the sages differ from you, saying: "Until midnight," specifically, and not afterwards? (and "one against many, the halachah is according to the many"), or do the sages hold with you, but say "until midnight" to keep one far from transgression? And he answered: The sages hold with me, and say: "Until midnight" to keep one far from transgression; and you are obliged to recite it.], and they said to him: We did not yet recite the Shema. He told them: If the morning star has not yet appeared, it is incumbent upon you to recite it. And not this alone [(This is still R. Gamliel speaking to his sons)] did they say, but wherever the sages say "until midnight," the mitzvah obtains until the appearance of the morning star. The mitzvah of burning the fats [of the offerings] and the pieces [of the daily afternoon burnt-offering — It is a mitzvah to offer up the pieces the entire night, viz. (Leviticus 6:2): "It is the burnt-offering upon its firewood on the altar all the night until the morning."] obtains until the appearance of the morning star. And the mitzvah of eating all of those offerings which must be eaten in one day [such as a thank-offering, a sin-offering, a guilt-offering, and the like, which are eaten a day and a night — the time for eating them is until the appearance of the morning star; and it is that which brings them to the status of nothar (left beyond the designated time and requiring to be burned)], ("the mitzvah, etc.") obtains until the appearance of the morning star. If so, why did the sages say: "Until midnight"? [in respect to the recital of the Shema and the eating of offerings. But they did not say "until midnight" at all in respect to the burning of the fats and the pieces, having mentioned this here only to apprise us that the mitzvah of all things designated for the night obtains the entire night.] To keep one far from transgression. [that he not come to eat them after the appearance of the morning star and incur kareth ("cutting-off"); and, similarly, with the recital of the Shema, that he not say "I still have time" and miss the designated time].