Tractate Soferim

It is not permitted to write [the sacred] books, tefillin and mezuzoth 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 [all these] shall be written on the skins of ritually clean cattle or ritually clean wild animals, be sewn together with their sinews and [the tefillin rolls] wound with their hair. It is also an oral prescription delivered to Moses at Sinai that [the parchments] shall be ruled with a reed and the writing on the skin be done with ink. [This latter requirement] has support in Scripture, And I wrote them with ink in the book.

R. Joshua the grits-dealer was asked: ‘Why is it permitted to write [sacred scrolls] on the skins of nebeloth and ṭerefoth?’ ‘I will give you an analogy,’ R. Joshua replied. ‘This is similar to the case of two men who were condemned to death by the State, one being executed by the king and the other by the executioner. Which of them is superior? He who was executed by the king.’ ‘If so,’ he retorted, ‘they should be permitted to be eaten!’ ‘It is stated,’ replied R. Joshua, ‘Ye shall not eat any nebelah. Scripture has forbidden it, so what can I do for you?’ (From here it is inferred that it is permitted to write on skins of nebelah and on skins of ṭerefah and there is no need to consider the possibility of their having been pierced at the animal’s heart.)