D'oraita and D'rabbanan: The Shema and Berakhot 1:3

(3) The school of Shammai says: In the evening all people should recline and recite [Shema], and in the morning they should stand, since it says [in the verse (Deut. 6:7)], “And when you lie down and when you arise.” But the school of Hillel says: Each person may recite it in his usual way (posture), since it says (ibid.), “And when you walk on the road.” If so, why does it say “and when you lie down and when you arise”? —[It means:] at the time when people are lying down, and at the time when people are arising. Said Rabbi Tarfon: “I was once traveling on the road, and I reclined to recite [Shema] in accordance with the view of the school of Shammai, and [by doing so] I put myself in danger of [attack by] bandits.” They [the other Sages] said to him: “You would have deserved to be guilty for your own fate, since you went against the view of the school of Hillel.”