Reference for Avodah Zarah 1:1
משנה לפני אידיהן של נכרים שלשה ימים אסורין מלשאת ומלתת עמהן מלהשאילן ומלשאול מהן מלהלוותן ומלהלוות מהן מלפורעם ומלהפרע מהן רבי יהודה אומר נפרעין מהן מפני שהוא מיצר אמרו לו אע"פ שהוא מיצר עכשיו שמח הוא לאחר זמן רבי ישמעאל אומר שלשה לפניהן ושלשה לאחריהן אסור וחכמים אומרים לפני אידיהן אסור לאחר אידיהן מותר:
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.