Parashas Terumah · Passuk 25:5
Shabbos 28b — the discussion establishes the restriction on materials used for sacred purposes and distinguishes tefillin from other sacred leather.
English: The Gemara derives by a kal vachomer from goat hair that a hide of an impure animal can become susceptible to corpse-tumah. It then asks about the teaching of Rav Yosef that only the hide of a pure animal is fit for sacred work. That teaching is applied to the leather of tefillin; the Gemara then discusses the halachah concerning the letter-shin on tefillin.
What Is Learned: The passage distinguishes the general laws of sacred materials from the special requirement for tefillin leather.