Shabbat 28B

אתיא בקל וחומר מנוצה של עזים, שאין מטמא בנגעים ⁠— מטמא באהל המת, עור בהמה טמאה שמטמאה בנגעים ⁠— אינו דין שמטמאה באהל המת.

It is derived through an a fortiori inference from goats’ hair. Although goats’ hair does not become ritually impure from tzara'at, it does become ritually impure as a tent over a corpse; with regard to the hide of a non-kosher animal that becomes ritually impure from tzara'at, is it not the case that it becomes ritually impure as a tent over a corpse?

ואלא הא דתני רב יוסף לא הוכשרו במלאכת שמים אלא עור בהמה טהורה בלבד ⁠— למאי הלכתא? לתפילין. תפילין בהדיא כתיב בהו ״למען תהיה תורת ה׳ בפיך״, מן המותר בפיך!

Since the conclusion was that the hide of even a non-kosher animal can become ritually impure as a tent over a corpse, it is not necessary to assume that the covering of the Tabernacle was made specifically from the hide of a kosher animal. And, if so, that which Rav Yosef taught: Only the hide of a kosher animal was suitable for heavenly service, for what halakha is that relevant, as it is clearly not relevant to the Tabernacle? The Talmud replies: This halakha was stated with regard to tefillin, which may be prepared only from the hide of a kosher animal. The Talmud asks: tefillin? Why did Rav Yosef need to state that halakha? It is written explicitly with regard to them: “And it should be for a sign to you upon your hand, and for a memorial between your eyes, that the law of YHWH may be in your mouth” (Exodus 13:9). The rabbis derived from there that the tefillin must be prepared from that which is permitted to be eaten in your mouth.

אלא לעורן. והאמר אביי: שין של תפילין הלכה למשה מסיני!

Rather, the Talmud explains that this halakha of Rav Yosef was said only with regard to the leather of the boxes that house the tefillin, which must be crafted from the hide of a kosher animal. It was not referring to the parchment on which the portions of the Torah inserted into the tefillin are written. The Talmud asks: Didn’t Abaye say: The obligation to make a letter shin protruding on the tefillin of one’s head is a halakha transmitted to Moses from Sinai? Since Torah law addresses the boxes of the tefillin, presumably their legal status is parallel to that of the parchment and the prohibition against preparing them from the hide of a non-kosher animal is by Torah law as well.