Tractate Oholot - Chapter 6 - Mishnah 4 אהלות פרק ו׳ משנה ד׳

[In the case of] a wall between two houses and there is uncleanness within it, The house nearer to the uncleanness is unclean, And the house nearer to the clean part is clean. If [the uncleanness] is in the middle, both are unclean. If there is uncleanness in one of the [houses] and there are vessels in [the thickness of] the wall: Those in the half nearer the uncleanness are unclean, Those in the half nearer the clean [house] are clean, And those in the middle are unclean. [With regard to the] plaster-work between the house and the upper story, and there is uncleanness in it: If it is in the lower half, the house [below] is unclean and the upper story is clean If it is in the upper half, the upper story is unclean and the house is clean: If it is in the middle, both are unclean. If there is uncleanness in either [the house or the upper story] and there are vessels inside the plaster-work, Those in the half nearer the uncleanness are unclean, And those in the half nearer the clean [space] are clean. If they are in the middle, they are unclean. Rabbi Judah says: all the plaster-work is considered as part of the upper story.