The Mishnah in Berakhot 9b records that according to the first opinion the morning Shema must be finished by sunrise, and the Gemara there teaches that the vatikim would time their recitation of Shema to conclude precisely at sunrise so as to adjoin redemption to prayer; Berakhot 8b adds that one may in fact recite Shema twice, once before and once after sunrise, fulfilling both the day and night obligations, while Berakhot 9a explains that the pre-dawn hour is itself already considered day though people are still commonly asleep then. The Tosefta Berakhot (Lieberman) 1:2-4 likewise states that the mitzvah of Shema is to be timed with sunrise so that prayer follows immediately after, illustrating this with Rabbi Yehuda's account of accompanying Rabbi Akiva and Rabbi Elazar ben Azarya until the sun was visible over the mountaintops.
The Rambam in Mishneh Torah, Prayer and the Priestly Blessing 3:1-6 rules that the mitzvah of Shacharit is to begin precisely at sunrise, extending until the end of the fourth hour, and that praying after that point, while valid after the fact, forfeits the rabbinic requirement of praying בזמנה. The Tur, Orach Chayim 89 and Beit Yosef, Orach Chayim 89:1 confirm that the time for Shacharit begins once dawn breaks and the eastern sky has lightened, though the primary mitzvah remains sunrise, and that one who prayed from that earlier point has fulfilled the obligation; the Arukh HaShulchan, Orach Chaim 89:11-13 cites the same formulation from the Beit Yosef and the Rosh that the time begins when the first light of dawn appears in the east.
Regarding the parallel question of Krias Shema, the Shulchan Arukh, Orach Chayim 58:1-3 and Tur, Orach Chayim 58 rule that the time begins when one can recognize a slight acquaintance at four amos, continuing until the end of three hours, with the most praiseworthy fulfillment being to time the reading as the vatikim did, finishing just before sunrise so that prayer follows immediately at sunrise. The Mishnah Berurah 58:2 explains that Chazal fixed this time of recognizing one's friend rather than the Scriptural standard of arising because the mitzvah of tzitzit embedded in the Shema requires sufficient light to see, and the Mishnah Berurah 89:1-2 cautions that one should wait to pray Shacharit until sunrise even when coming early to shul, though in cases of pressing need such as travel one may be lenient, while still waiting to recite Shema itself until the time of recognizing a friend.
Peninei Halakhah, Prayer 11:1 clarifies that amud hashachar, the first halachically significant moment of the day, permits Shema and the Amidah only under extenuating circumstances, while under normal conditions this is prohibited, and that the Acharonim dispute the exact point when this dawn light begins; Peninei Halakhah, Prayer 11:5 and Peninei Halakhah, Prayer 11:6 elaborate that the praiseworthy practice of the vatikim is to recite Shema just before sunrise and begin the Amidah precisely at sunrise, while noting that poskim disagree about the exact moment of netz itself.
The Shulchan Arukh HaRav, Orach Chayim 89:9 permits one who must depart early for travel to pray the Amidah from the time dawn has broken, before sunrise, reciting Shema together with its blessings at home if enough light has come for the time of Shema, or praying the Amidah alone first if it has not; this is echoed by the lenient treatment in Kitzur Shulchan Arukh Yalkut Yosef, Orach Chaim 89:6, which permits one who cannot find a minyan within the stricter four-hour reckoning to rely on a more lenient counting of the hours from dawn so as not to lose תפלה בציבור.
Netivot Olam, Netiv Ha'Avodah 3:3 explains the deeper reason for fixing prayer at sunrise, that since sunrise marks the beginning of the day, it is the fitting time for a person, likened to an effect bound to its cause, to attach himself to Hashem, his ultimate source, just as mincha is tied to sunset as the day's completion; this is reinforced by verses such as Tehillim 5:4 describing calling out to Hashem at daybreak and Yeshayahu 26:9 describing yearning for Him through the night and dawn.
Berakhot 14a establishes the broader prohibition against attending to other matters, including greeting a friend or tending to personal affairs, before one has prayed, reinforcing the urgency of not delaying tefillah unnecessarily, a concern echoed in Mishnah Berurah 281:1 and Mishnah Berurah 250:1, which caution against letting the time for Shema and tefillah pass even amid other obligations.
Among the Acharonim addressing related edge cases, Mishnah Berurah 235:29-31 discusses one who is prevented (אנוס) from reciting the evening Shema and may do so even after dawn so long as it is before sunrise, and Mishnah Berurah 52:3 and Mishnah Berurah 52:5 address the practical order of pesukei dezimra so that one may still reach the Amidah together with the congregation.
Among modern poskim, Maamar Mordechai Limot HaChol 13:2 rules that one who has stayed awake all night, such as on Shavuot or Hoshana Rabba, should pray Shacharit at sunrise, or if that is difficult may pray from alos hashachar provided he does not pray before that point, and Maamar Mordechai Limot HaChol 13:4 addresses the practical measure of how close to sunrise the chazzan's completion of Shema must be to count as being said with the sunrise, and permits an individual accustomed to praying at sunrise who finds no minyan at that hour to pray alone. Shut HaRav HaRashi 5:17 weighs, for laborers who must rise early, whether it is preferable to pray a fuller service at or near sunrise or to pray hastily after sunrise with the congregation, concluding that praying somewhat before or at sunrise so as to stay with the group is permitted.
Teshuvos V'hanhagos (Teshuvos V'hanhagos 6:25) addresses that according to the Rambam the ideal time for the morning Shema begins at the very start of netz hachamah, when the sun's first point appears over the horizon, and continues through the whole duration of sunrise.
Az Nidberu (Az Nidberu 11:26) takes up the question of whether the principle that the zealous act early is preferable, or whether praying close to sunrise is preferable as analogous to praying like the vatikim rather than before sunrise.
Tzitz Eliezer (Tzitz Eliezer 13:60) holds that the period from dawn until sunrise serves as the time for the morning Shema, and also as an opportunity for one who was prevented from reciting the evening Shema to fulfill that obligation before sunrise.
Tzitz Eliezer (Tzitz Eliezer 14:5) explains that although one who recites Shema after sunrise fulfills the obligation through the end of the third hour if delayed, the Rambam treats praying precisely at sunrise as an initial legal requirement rather than merely a preferred practice, and Az Nidberu (Az Nidberu 11:25) separately takes up a practical case of two neighborhood synagogues both praying Shacharit before sunrise but at differing times.
Igros Moshe (Igros Moshe OC 4:6) permits, in a case of severe necessity where work obligations do not allow waiting, reciting Shema and praying from alos hashachar, while recommending that one still don tallit and tefillin, and distinguishing the earlier time permitted for prayer from the later time required for Shema.