Rav said to his uncle, Rabbi Ḥiyya: I did not see Rabbi Yehuda HaNasi accept the kingship of Heaven upon himself, meaning that he did not see him recite Shema. In response Rabbi Ḥiyya said to him, when Rabbi Yehuda HaNasi passed his hands over his face in the study hall in the middle of his lesson, he accepted the yoke of the kingdom of Heaven upon himself, as his Shema was comprised of a single verse.
Rabbi Yehuda HaNasi’s students and members of his household disputed:
Does he complete Shema later or does he not complete it later? Bar Kappara says: He does not complete it later. Rabbi Shimon, son of Rabbi Yehuda HaNasi, says: He completes it later. Bar Kappara said to Rabbi Shimon, son of Rabbi Yehuda HaNasi:
Granted, according to my position, that I say that Rabbi Yehuda HaNasi does not complete Shema later, that is why when he taught, Rabbi Yehuda HaNasi would specifically seek a topic that included the exodus from Egypt, as by so doing he fulfills the mitzva to remember the Exodus; a mitzva that others fulfill in their recitation of the last paragraph of Shema. But according to you, who says that he completes his recitation of Shema later, why, when he teaches, would he specifically seek a topic that included the exodus from Egypt?
Rabbi Shimon responded: Rabbi Yehuda HaNasi did so in order to mention the exodus from Egypt at its appointed time, during the time of the recitation of Shema.
Although this Gemara is describing an unusual situation, where Rabbi Yehuda HaNasi fulfilled the mitzva of Shema while in the middle of his lecture, his practice of passing his hand over his face has become a standard feature of the recitation of the Shema. Various explanations of this practice have been offered. Rav Hai Ga’on and the Aruk suggest that while accepting God’s dominion over the entire world, one must direct his eyes in all directions, and Rabbi Yehuda HaNasi covered his eyes to conceal their movement. Rashi explains that passing his hands over his face was meant to keep all potential distractions from his sight in order to facilitate proper intent.