Reference for Bava Batra 201:3
ועושה חצר על פי המערה שש על שש כמלא המטה וקובריה
R. SIMEON SAYS: THE CENTRAL SPACE OF THE GROTTO MUST CONTAIN [AN AREA OF] SIX CUBITS BY EIGHT. AND THIRTEEN CHAMBERS ARE TO OPEN OUT INTO IT; FOUR ON ONE SIDE, FOUR ON THE OTHER. THREE IN FRONT [OF THE ENTRANCE]. ONE ON THE RIGHT OF THE ENTRANCE AND ONE ON THE LEFT.<span class="x" onmousemove="('comment',' According to R. Simeon, the longer walls, being eight cubits in length. provide space for four one-cubit chambers each, allowing three cubits for the intervening one-cubit walls, and one cubit space for the two half cubit spaces at the corners. The wall opposite the entrance, being six cubits in length, can contain three one-cubit chambers, the space for the two one-cubit intervening walls and the two half-cubit spaces at the corners. This gives a total of, (4 + 4 + 3), eleven sepulchral chambers. The location of the last two is dealt with in the Gemara infra. ');"><sup>3</sup></span>