Yosef Karo, Translation:Shulchan Aruch — Orach Chaim 15Transferring Fringes between Garments and Laws For a…
“ And Rav Amram explains that the reason [one should not sew up the tallit] is because a tear within 3 [fingerbreadths] it [the torn corner] does not come into the realm of having the status of a garment, and it's compared to [a garment] that has no [measurement] , and even if was sewed back on, it is considered that it was already set [in its status] and if one were to make tzitzit for it, it would not exempt the tallit. ”
