Talmud for Bava Batra 2:2
וכן בגינה מקום שנהגו לגדור יגדור אבל בבקעה מקום שנהגו שלא לגדור אין מחייבין אותו אלא אם רצה כונס לתוך שלו ועושה חזית מבחוץ לפיכך אם נפל הכותל המקום והאבנים שלו ואם עשו מדעת שניהם בונים את הכותל באמצע ועושים חזית מיכן ומיכן לפיכך אם נפל הכותל המקום והאבנים של שניהם:
It teaches in a Baraita: "The one who recites [the Shema] before this has not fulfilled their obligation. If so, why do we recite it [before nightfall] in the synagogue? Rabbi Yose said: 'We do not recite it in the synagogue to fulfill its obligation. Rather, [we recite it] so that we may stand up in prayer after a word of Torah." Rabbi Zeira in the name of Rav Yirmiyah: "If there is doubt whether he made the blessings after his food or not, he should make the blessings [even though he may be doing it a second time]. For it is written (Deut. 8:10) 'And you shall eat, and you shall be satisfied, then you shall bless [the LORD your God...]' If there is doubt whether he prayed or not, he should not pray [as he may have already done so.]" But he disagrees with Rabbi Yohanan. For Rabbi Yohanan said: "If only a person could pray the entire day!" Why? For prayer is never a waste. If there is doubt whether he recited [Shema] or not, we may gather from this baraita: "The one who recites [the Shema] before this has not fulfilled their obligation." And is not before [nightfall] a time of doubt? And yet you say "he should recite [Shema]." This implies that if there is doubt whether he recited [Shema] or not, he should recite [Shema]. [From Tosefta 1:1] "The indicator of this is when the stars come out. And though there is no proof of it [from Scripture], there is a trace of it: (Neh. 4:21) '...we were doing the labor, and half of them were holding the spears from the break of dawn until the stars came out.' And he writes: '...it is guard-time for us by night and labor by day." How many stars should come out so that it becomes night? Rabbi Pinhas in the name of Rabbi Abba bar Papa: "One star, surely day. Two, doubt. Three, surely night." "...two doubt," [really]? But is it not written, "...until the stars came out!?" Rather, [the plural] "stars" indicates two! [Not three!]--The first one doesn't count. [An unattributed baraita:] "If he sees one star on the eve of the Sabbath and performs a creative task, he is exempt [from punishment]. Two, he brings a conditional guilt offering, Three, he brings a sin offering. On the departure of the Sabbath, if he sees on star and performs a creative task,