TT #139 · Parshas Tazria · Aliyah 2 · Passuk 14
English: But as soon as raw flesh appears on them, they shall be impure.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:14
Torah Temimah cites Sifra (Toras Kohanim) for the lentil-sized measure for the healthy-flesh sign.
Sifra — Vayikra 13, Midrash Sefer
English: The “living flesh” mentioned here is compared with the living flesh above: just as there it is the size of a lentil, so it is here.
Textual derivation
The healthy-flesh sign uses the same minimum size measure taught for healthy flesh earlier: an area the size of a lentil.