Ezekiel 42:3, 19-20

Both in the section twenty cubits from the inner court and in the section opposite the pavement of the outer court, gallery faced gallery at the three levels.

Then he turned to the west side and measured; it was five hundred cubits by the measuring rod. So he measured the area on all four sides. It had a wall around it, five hundred cubits long and five hundred cubits wide, to separate the holy from the common.