TT #183 · Parshas Tazria · Aliyah 5 · Passuk 28
English: If the bright spot has not spread in the skin and is faded, it is the swelling from the burn, and the priest shall pronounce him clean, for it is the scar of the burn.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:28
Torah Temimah cites Sifra (Toras Kohanim) for the crusted stage required for a burn scar.
Sifra — Vayikra 13, Midrash Sefer
English: The burn scar reaches the required stage when it crusts like the peel of a garlic clove.
Textual derivation
The Sifra defines the healing stage of the burn scar.