Berakhot 13b:18ברכות י״ג ב:י״ח
The Sages taught in a baraita: The single verse, “Hear, Israel, the Lord is our God, the Lord is One”; this is Rabbi Yehuda HaNasi’s recitation of Shema. The Gemara relates: 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. Rabbi Ḥiyya said to him: Son of noblemen [bar paḥtei], 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.
Rosh on Berakhot 2:3:1פסקי הרא"ש על ברכות ב׳:ג׳:א׳
...
Beit Yosef, Orach Chayim 61:5:1בית יוסף, אורח חיים ס״א:ה׳:א׳
...
Shulchan Arukh, Orach Chayim 61:5שולחן ערוך, אורח חיים ס״א:ה׳
The custom is to place one's hands over their faces during the recitation of the first verse in order that one will not look at something else that will prevent him from intending [his heart].
Arukh HaShulchan, Orach Chaim 61:3ערוך השולחן, אורח חיים ס״א:ג׳
...