Leviticus 13:26-27

But if, after looking at it, the priest sees that there is no white hair on the bright place, and it is not deeper than the skin, and is not very bright, then let the priest keep him shut up for seven days: And the priest is to see him again on the seventh day; if it is increased in the skin, then the priest will say that he is unclean: it is the leper's disease.