Mishnà su Levitico 13:33

How does one shave who has a scall? He shaves outside it and leaves next to it two hairs in order to determine whether it spreads. If he was certified unclean on account of yellow hair, and then the yellow hair disappeared and other yellow hair appeared, and so also if there was a spreading, irrespective of whether the certification took place at the beginning, at the end of the first week, at the end of the second week or after his release from uncleanness, behold he remains as he was before. If he was certified unclean on account of a spreading, and the spreading disappeared and then reappeared, and so also if there was yellow hair, irrespective of whether the certification took place at the end of the first week, at the end of the second week or after his release from uncleanness, he remains as he was before.