Midrash sur Le Lévitique 14:37

1) (Vayikra 14:37) ("And he shall see the plague-spot, and, behold, if the plague-spot is in the walls of the house, embedded (sheka'aroroth) deep green or deep red, and their appearance is lower than the wall,") "And he shall see the plague-spot": I might think (if he sees it to be the minimum size of) a garis; it is, therefore, written "And he shall see the plague-spot and behold, the plague-spot." We are hereby taught that the house does not become tamei with less than two garisin.

1) (Vayikra 14:43) ("And if the plague-spot returns and blossoms in the house after he removed the stones and after the house has been scraped and after it has been plastered. (Vayikra 14:44) And the Cohein shall come and he shall see, and, behold, the plague-spot has spread in the house, it is blight leprosy in the house; it is tamei.") "And if the plague-spot returns and blossoms in the house": Just as we speak of a man returning to his place, here, too, the plague-spot returns to the same stones. This tells me only of his place. Whence do we derive the entire house for inclusion? From "in the house." I might think that the size of a garis (makes it subject to tumah); it is, therefore, written here "plague-spot" and before, (Vayikra 14:37), "plague-spot." Just as there, two garisin, here, too, two garisin.

2) "in the walls": implying two. And below, (Vayikra 14:39), it is written [(instead of)] "wall," "walls," giving four, whereby we are taught that it becomes tamei only if it has four walls.