Reference for Makkot 21:8

למ"ד פסול דכתיב (שמות יג, ט) למען תהיה תורת ה' בפיך ואיתקש כל התורה כולה לתפילין מה תפילין הלכה למשה מסיני לתופרן בגידין אף כל לתופרן בגידין ואידך כי איתקש למותר בפיך להלכותיו לא איתקש

According to the opinion that it is invalid [the reason is] that the Torah says (Exodus 13, 9) "In order that the Torah of God should be in your mouth" which compares the whole Torah to Tefillin. Just as Tefillin is a law from Moshe at [Mt.] Sinai to tie them with sinews, so too the whole [Torah] has to be tied with sinews. And the other [opinion holds], in what aspect was it compared? That it must be [from an animal] permitted to the mouth. For the rest of it's laws, it wasn't compared.

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