Midrash su Levitico 13:57

1) (Vayikra 13:56) ("And if the Cohein saw, and, behold, the plague-spot became dim after it was washed, then he shall tear it from the garment, or from the skin, or from the warp, or from the woof, (Vayikra 13:57). And if it be seen again in the garment, or in the warp, or in the woof, or in any article of skin, it is porachath. In fire shall you burn it, what contains the plague-spot.") "then he shall tear": I might think that he tears it just a little to fulfill the mitzvah of tearing; it is, therefore, written "it." If "it," I might think that he might slash it and leave it in its place; it is, therefore, written "from the garment" — he must remove it from the garment.

4) (Vayikra 13:57) ("And if it be seen again in the garment, or in the warp, or in the woof, or in any article of skin, it is porachath (something whose "blossoming" recurs). In fire shall you burn it (the entire garment), what contains the plague-spot"). Whence is it derived that he places a patch upon it (the site of the torn out plague-spot)? From "and if it be seen again," and "again" can apply only to the site," whereby we learn that he places a patch upon it.

5) R. Nechemiah says: A patch is not required, ("again" referring to the garment from which the plague-spot was torn out). If (after the patch was placed on the site) the plague-spot returned to the garment (at a different site), the patch is "rescued" (i.e., it need not be burned, not being part of the original garment). If it returned upon the patch, the (entire) garment is burned (along with the patch). If one patched aught of a quarantined (garment) on a clean (garment), and the plague-spot reappeared on the (clean) garment, the patch is burned. If it reappeared on the patch, the first garment is burned. And (a separate ruling:) The patch can be used for the second garment with (identifying signs [so that it not be confused with other patches]).