Tractate Sefer Torah

It is not permitted to write a scroll of the Torah on skins of ritually unclean cattle or on skins of ritually unclean wild animals, nor may they be sewn with [their] sinews or [the parchment rolls of tefillin] be wound with their hair. It is an oral prescription delivered to Moses at Sinai that [these] shall be written on the skins of ritually clean cattle, or on skins of ritually clean wild animals, that they shall be sewn together with ritually clean sinews, [that the tefillin rolls] shall be wound with ritually clean hair, and that the writing be done with ink, because [this latter requirement] has support in Scripture, And I wrote them with ink in the book.

It is permitted to write [sacred scrolls] on skins of nebeloth and on skins of ṭerefoth and there is no need to consider the possibility of their having been pierced at the animal’s heart.

Skins for the [sacred] books, tefillin and mezuzoth may be bought anywhere, and one need not consider the possibility that they might be skins that were pierced at the animal’s heart.