Mishnah 3

בֵּית שַׁמַּאי אוֹמְרִים: בָּעֶרֶב כָּל אָדָם יַטּוּ וְיִקְרְאוּ, וּבַבֹּקֶר יַעַמְדוּ, שֶׁנֶּאֱמַר: "וּבְשָׁכְבְּךָ וּבְקוּמֶךָ". וּבֵית הִלֵּל אוֹמְרִים: כָּל אָדָם קוֹרֵא כְדַרְכּוֹ, שֶׁנֶּאֱמַר: "וּבְלֶכְתְּךָ בַדֶּרֶךְ". אִם כֵּן לָמָּה נֶאֱמַר "וּבְשָׁכְבְּךָ וּבְקוּמֶךָ"? בְּשָׁעָה שֶׁבְּנֵי אָדָם שׁוֹכְבִים, וּבְשָׁעָה שֶׁבְּנֵי אָדָם עוֹמְדִים. אָמַר רַבִּי טַרְפוֹן: אֲנִי הָיִיתִי בָא בַדֶּרֶךְ, וְהִטֵּיתִי לִקְרוֹת כְּדִבְרֵי בֵית שַׁמַּאי, וְסִכַּנְתִּי בְעַצְמִי מִפְּנֵי הַלִּסְטִים. אָמְרוּ לוֹ: כְּדַי הָיִיתָ לָחוּב בְּעַצְמְךָ, שֶׁעָבַרְתָּ עַל דִּבְרֵי בֵית הִלֵּל.

The school of Shammai says: in the evening all people should recline and recite [the Shema], and in the morning they should stand, since it says [in the verse (Deut. 6:7)], “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.), “when you walk on the road”. If so, why does it say “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. Rabbi Tarfon said: “I was once traveling on the road and I reclined to recite [the 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.”