Berakhot 24A

כל לנטורינהו טפי עדיף. והיכא מנח להו? אמר רבי ירמיה: בין כר לכסת, שלא כנגד ראשו.

Because whatever offers more protection is preferable even at the cost of deprecation. And where under his head does he place them? R' Yirmeya said: He places them between the pillow and the mattress, not directly aligned with his head but rather a bit to the side.

והא תני רבי חייא: מניחן בכובע, תחת מראשותיו! דמפיק ליה למורשא דכובע לבר.

The Talmud asks: Didn’t R' Ḥiyya teach a baraita that in that case he places them in a pouch used for tefillin, directly under his head? The Talmud replies: He does so in a manner that the bulge in the pouch, where the tefillin are, protrudes out and is not beneath his head.

בר קפרא צייר להו בכילתא, ומפיק למורשהון לבר. רב שישא בריה דרב אידי מנח להו אשרשיפא, ופריס סודרא עילוייהו.

On this note, the Talmud relates that Bar Kappara would tie them in his bed curtain and project their bulge outward. Rav Sheisha, son of Rav Idi, would place them on a bench and spread a cloth over them.

אמר רב המנונא בריה דרב יוסף: זימנא חדא הוה קאימנא קמיה דרבא, ואמר לי: זיל אייתי לי תפילין. ואשכחתינהו בין כר לכסת, שלא כנגד ראשו. והוה ידענא דיום טבילה הוה, ולאגמורן הלכה למעשה הוא דעבד.

Rav Hamnuna, son of Rav Yosef, said: I was once standing before Rava and he told me: Go and bring me my tefillin. And I found them in his bed, between the mattress and the pillow, not aligned with his head. And I knew that it was the day of his wife’s immersion in the mikveh for purification from the ritual impurity of a menstruating woman, and he certainly engaged in marital sex in order to fulfill the mitzva, and he did so, he sent me to bring him his tefillin, to teach us the practical halakha in that case.

בעא מיניה רב יוסף בריה דרב נחוניא מרב יהודה: שנים שישנים במטה אחת, מהו שזה יחזיר פניו ויקרא קריאת שמע, וזה יחזיר פניו ויקרא קריאת שמע? אמר ליה: הכי אמר שמואל: ואפילו אשתו עמו.

Rav Yosef, son of Rav Neḥunya, who raised a dilemma above, raised a dilemma before Rav Yehuda: Two individuals sleeping in a single bed, given that it was standard practice to sleep without clothing, what is the halakha; is it permissible for this one to turn his head aside and recite Shema and for that one turns his head and recites Shema; or is it prohibited because they are unclothed and are considered unfit to recite Shema even though they are covered with a blanket? He said to him: Shmuel said as follows: This is permitted even if his wife is in bed with him.