Mishnayos Brachos Perek 1 Mishnah 3

Beit Shammai and Beit Hillel disputed the proper way to recite Shema. Beit Shammai say: One should recite Shema in the manner indicated in the text of Shema itself. Therefore, in the evening every person must recline on his side and recite Shema, in fulfillment of the verse: “When you lie down,” and in the morning he must stand and recite Shema, in fulfillment of the verse: When you rise, as it is stated: “When you lie down, and when you rise.” And Beit Hillel say: Every person recites Shema as he is, and he may do so in whatever position is most comfortable for him, both day and night, as it is stated: “And when you walk along the way,” when one is neither standing nor reclining (Me’iri). If so, according to Beit Hillel, why was it stated: “When you lie down, and when you rise”? This is merely to denote time; at the time when people lie down and the time when people rise. With regard to this halakha, Rabbi Tarfon said: Once, I was coming on the road when I stopped and reclined to recite Shema in accordance with the statement of Beit Shammai. Although Rabbi Tarfon was a disciple of Beit Hillel, he thought that fulfilling the mitzva in accordance with the opinion of Beit Shammai would be a more meticulous fulfillment of the mitzva, acceptable to all opinions. Yet in so doing, I endangered myself due to the highwaymen [listim] who accost travelers. The Sages said to him: You deserved to be in a position where you were liable to pay with your life, as you transgressed the statement of Beit Hillel. This statement will be explained in the Gemara.