Berachos, Chapter 1, Mishnah 3. After the previous mishnayos discussed the times for reciting Shema, the mishnah presents a fascinating dispute between Beis Shammai and Beis Hillel about the position a person must be in while reciting Shema.
The Two Approaches:
Beis Shammai: In the evening a person lies down and recites the Shema, and in the morning he stands and recites it, following the wording of the verse "beshochbecha uvekumecha" - "when you lie down and when you rise." According to their approach, the Torah is not teaching only about the time of the recitation, but also about the physical position of the one reciting.
Beis Hillel: A person recites in his usual manner and in whatever position he happens to be in, and even while walking along the road he may recite as he walks. And what, then, does the verse "uveshochbecha uvekumecha" mean? It refers to the time when people generally lie down to sleep, and to the time when they generally rise.
An Incident That Occurred:
Rabbi Tarfon relates: "hayisi mehalech baderech vehiteisi likros krias Shema" - "I was walking along the road and I reclined in order to recite the Shema." He lowered himself to lie down in accordance with the opinion of Beis Shammai, that one must recite while lying down. Because he was in a reclining position, he placed himself in danger from bandits, and at that moment he was unable to defend himself.
The Sage said to him: "kedai hayisa lachov be'atzmecha" - "you deserved to bring harm upon yourself," for you transgressed the words of Beis Hillel, and you are obligated to rule like Beis Hillel; permission is not granted to rule like Beis Shammai.
In Summary: In this mishnah we learned that according to Beis Shammai the verse "when you lie down and when you rise" teaches about the position as well - lying down in the evening and standing in the morning - while according to Beis Hillel the verse is merely an indication of time, and a person recites in whatever position he happens to be in. From the incident with Rabbi Tarfon, who reclined in order to recite according to the opinion of Beis Shammai and thereby placed himself in danger from bandits, we learn that the halachah follows Beis Hillel, and one who acts like Beis Shammai deserves to have brought harm upon himself.