Mishnah for Hallah 1:1
משנה חמשה דברים חייבין בחלה החיטים והשעורין והכוסמין ושיבולת שועל והשיפון הרי אלו חייבין בחלה. ומצטרפין זה עם זה ואסורים בחדש מלפני הפסח ומלקצור מלפני העומר ואם השרישו קודם לעומר העומר מתירן ואם לאו אסורין עד הבא העומר הבא:
From when may one recite Shema in the evening? From the time the Kohanim go in to eat their Terumah (produce consecrated for priestly consumption). Until the end of the first watch, says Rabbi Eliezer. And the Sages say: Until [astronomical] midnight. Rabban Gamliel says: Until the break of dawn. It once happened that his [Rabban Gamliel’s] sons came from a house of feasting. They said to him: We have not recited Shema. He to them: If dawn has not broken, you are obligated to recite it. And it is not only in this case that they said it! Rather, in all cases where the Sages said "only until midnight," the obligation remains until the break of dawn. [e.g.] Burning the fats and limbs [of the sacrifices, on the Temple altar] — the obligation is until the break of dawn. [e.g.:] All [sacrifices] which may be eaten for one day — the obligation is until the break of dawn. If that is so, why did the Sages say, "until midnight?" To distance a person from transgression.