Mishnah Middot
The [chamber] of the lowest [story] was five cubits wide and at the ceiling six cubits. The [chamber] of the middle [story] was six cubits wide and at the ceiling of seven. The [chamber] of the top [story] was seven cubits wide, as it says, "The lowest story was five cubits wide, the middle one 6 cubits wide and the third 7 cubits wide" (I Kings 6:6).