פירוש על מנחות 80:9

But doesn’t Rabbi Zeira say, "If one affixed [tsitsit] to a garment that already had them affixed, it is fit": It is a wonder: How might you think that Rabbi Zeira does not hold of, "Prepare [it], and not from what has already been prepared." Behold, Rabbi Zeira himself holds from it above! And it can be said (the answer is) that you might have thought that since the beginning of its placement was for the sake of the commandment, "Prepare it, and not from what has already been prepared," does not apply. But above, when it was cut, the [status of the] placement was nullified.