Leviticus 13:31

And if the priest look on the plague of the scall
As it may appear in another person, brought to him for inspection and examination: and, behold, it [be] not in the sight deeper than the skin;
it do not seem to be got into the flesh, or lower than the skin: and [that there is] no black hair in it;
or, "but no black hair in it"; for, as Jarchi says, if there was a black hair in it, he would be clean, and there would be no need of shutting up; for black hair in scalls is a sign of cleanness, as it is said, ( Leviticus 13:37 ) ; it would be a clear case that such a man had no leprosy on him; for black hair is a token of a strong and healthful constitution; and there could remain no doubt about it, and it would require no further trial and examination: Ben Gersom says it means two black hairs; and further observes, that black hair in the midst of a scall is a sign of cleanness; but this being wanting, then the priest shall shut up [him that hath] the plague of the scall
seven days;
from the time of his viewing the scall; and so Ben Gersom, this is the seventh day from the time of looking upon the scall.