Reference for Bava Batra 197:1

הא קא משמע לן למטה כלמעלה מה למעלה אין משמש כלום אף למטה אין משמש כלום

— It teaches us this: [The space] below<span class="x" onmousemove="('comment',' The ten cubits from the ground where the Cherubim and the Ark were standing. ');">¹</span> [was] as [that] above. As [the space] above<span class="x" onmousemove="('comment',' The space of twenty cubits from the Cherubim to the top. ');">²</span> served no [material] purpose.<span class="x" onmousemove="('comment',' They were empty. ');">³</span>

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