Reference for Shabbat 73:11
אמר ר' יצחק בר נחמני א"ר אושעיא קטמה והובערה משהין עליה חמין שהוחמו כל צורכן ותבשיל שבישל כל צורכו
on the inside and top thereof it is forbidden, but leaning against it may be permitted; or perhaps, there is no difference? — Come and hear: If two stoves are joined, one being swept and covered with ashes, whilst the other is neither swept nor covered with ashes: we may keep [aught] upon the one that is swept or covered with ashes, but not upon the one that is not swept or covered with ashes, though the heat reaches it from the other.<span class="x" onmousemove="('comment',' Our problem is similar, and this shows that it is permitted. ');"><sup>8</sup></span> Perhaps there it is different, because since it is elevated, the air affects it.<span class="x" onmousemove="('comment',' The pot stands on the stove and is surrounded by air, which cools it, and therefore the heat from the other stove is disregarded. But leaning against an unswept stove, without air interposing, may be forbidden. ');"><sup>9</sup></span> Come and hear: For R. Safra said in R. Hiyya's name: If it [the stove] was covered with ashes, yet blazed up again, one may lean [a pot] against it, keep [a pot) upon it, remove [it] thence and replace [it]. This proves that even leaning is [permitted] only when it is covered with ashes, but not otherwise. Yet according to your reasoning, when he states, 'one may remove [it] thence,'[does this imply] only if covered with ashes, but not otherwise?<span class="x" onmousemove="('comment',' Surely not! ');"><sup>10</sup></span> But [you must answer,] removing is mentioned on account of replacing; so here too, leaning is stated on account of keeping.<span class="x" onmousemove="('comment',' Yet covering with ashes may not be required for leaning. ');"><sup>11</sup></span> How compare! There, since removing and replacing refer to the same place, removing is stated on account of replacing; but here, the leaning is in one place whereas the keeping is in another! What is our decision thereon? — Come and hear: If a stove is heated with peat or wood, one may lean [a pot] against it, but must not keep [it there] unless it is swept or covered with ashes. If the coals have died down,<span class="x" onmousemove="('comment',' Not being entirely extinguished, but burning dully and feebly. ');"><sup>12</sup></span> or thoroughly beaten flax is placed upon it, it is as though covered with ashes.<span class="x" onmousemove="('comment',' Thus for leaning it need not be swept, etc. ');"><sup>13</sup></span> R. Isaac b. Nahmani said in R. Oshaia's name: If he covered it with ashes yet it blazed up again, one may keep upon it hot water that has [previously] been heated as much as is required, or a dish which has been boiled all it needs.