Reference for Zevachim 108:2
דמחית מידי מתותיה ושקיל ליה דאי לא תימא הכי הא דאמר רב כהנא אבנים של קרנות חלולות היו דכתיב (זכריה ט, טו) ומלאו כמזרק כזויות מזבח הכא נמי אבנים שלמות אמר רחמנא אלא דמחית מידי מתותיה ושקיל ליה ה"נ דמחית מידי מתותיה ושקיל ליה
were hollow, for it is written, And they shall be filled like the basins, like the horns of the altar,<span class="x" onmousemove="('comment',' Zech. IX, 15. - That implies that the horns were hollowed out to form a receptacle.');"><sup>5</sup></span> here too [you may object that] the Divine Law prescribed 'whole stones'?<span class="x" onmousemove="('comment',' Whereas if the horns were hollowed or perforated after they were made, the stones would have to be cut into.');"><sup>6</sup></span> But [you must answer] that something was [first] placed there<span class="x" onmousemove="('comment',' Thin laths formed the hollow or channels before the stones etc. were poured into it, and these were not filled in.');"><sup>7</sup></span> and then removed; so here too, planks were [first] placed there and then removed.