Saying Shema on time is greater than Learning Torah: Berachot 10b
General activities before Shema: Berachot 4b
Definition of "Unavoidable Circumstances" which permissibly cause one to miss the Extra Fence which the Rabbis instituted, of reciting Shema before Midnight: Rosh Berachot 1:9
Morning
Earliest time: Berachot 8b-9a, 9b
Latest time: Berachot 9b, 10b, 25b
Determining whether the time has begun: Berachot 3a, 9b
System of checking between Techelet-Dye and a White color: Berachot 9b [Rashi, Tos.]; Menachot 43b
The Best time, and the practice of the Vatikin: Berachot 9b, 25b, 26a; Tos. Berachot 9b #3