Talmud/Seder Zeraim/Tractate Berakhot/2a
(See the Mishnah for explanation)
Since when may one recite the Shema in the evening? Since the time when the kohanim (priests) enter to eat their terumah (agricultural gifts). [It may be recited at any time since then] until the end of the first watch [of the night]; this is the view of Rabbi Eliezer. The Sages say: [The Shema may be recited] until midnight. Rabban Gamliel says: [It may be recited] until dawn breaks (lit. the column of dawn rises).
It once happened that [Rabban Gamliel’s] sons came [home late at night] from a place of feasting. They said to [their father]: “We have not recited the Shema." He said to them: “If dawn has not broken, you are obligated to recite it.
"[This is true] not only in this case; rather, in all cases where the Sages said that [some precept can be performed only] until midnight — the precept is [still in force] until the break of dawn. [For example:] Burning the fats and organs [of the sacrifices, on the Temple altar] — this precept [can be performed] until the break of dawn. [Another example:] All [sacrifices] which may be eaten for one day — the precept [of eating them can be performed] until the break of dawn.
"If so, why did the Sages say [that these precepts can be performed only] until midnight? To keep a person away from sinning."
What does the Tanna refer to, asking "Since when"? Furthermore, why does he deal with [the Shema reading] of the evening first, rather than dealing with that of the morning first?
The Tanna refers to Scripture, which says, "…when you lie down and when you get up," and he means to say thus: What is the time of the reciting of the Shema at lying down? Since the time when the kohanim enter to eat their terumah.
And if you want, you can say that he learned [the order] from the creation of the world, thus: "…and it was evening and it was morning, day one."
If so, in the concluding Mishna, where it states: "In the morning one says two blessings before [the Shema] and one after it, while in the evening he says two blessings before it and two after it," he should have dealt with the morning one first!
The Tanna started with the evening one first, and then dealt with the morning one; while he was dealing with the morning one, he expounded the details of the morning one, and then expounded those of the evening one.
The master said, Since the time when the kohanim enter to eat their terumah.
Seeing that when do the kohanim eat terumah? – since the emergence of the stars, it should have stated, "since the emergence of the stars"!
It teaches us something "by the way": When do kohanim eat terumah? – since the time of the emergence of the stars; this teaches us that the atonement is not preclusive [of eating terumah], as it is taught [in Torat Kohanim Emor]: "And the sun would set and he would become [ritually] pure" (lit.: and it would be pure) (Lev. 22:7) – his sun-setting precludes him from eating terumah, but his atonement does not prevent him from eating teruma. How do we know that this [Scritural reference], "and the sun would set" (lit.: "and the sun would arrive") refers to sunset, and "…and it would be pure" means that the day would be cleared?