Reference for Zevachim 216:20

בעי רבי יוסי בר' חנינא קרן וכבש ויסוד וריבוע מהו שיעכבו בבמה

but there is no altar at the bamah;<span class="x" onmousemove="('comment',' Only at the door of the tent of meeting was a proper altar required. But when bamoth were permitted, no proper altar was necessary, and one could sacrifice and offer up on a simple stone.');">²⁴</span> therefore if one offered up [without] on a rock or on a stone, he is liable.

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