Mishnah for Nedarim 1:1
משנה כל כינויי נדרים כנדרים חרמים כחרמים שבועת כשבועות נזירות כנזירות האומר לחבירו מודר אני ממך מופרש אני ממך מרוחק אני ממך שאיני אוכל לך שאיני טועם לך אסור מנודה אני לך ר' עקיבה היה חוכך בזה להחמיר כנדרי רשעים נדר בנזיר בקרבן ובשבועה כנדרי כשרים לא אמר כלום כנדבותם נדר בנזיר ובקרבן:
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.