Reference for Bava Batra 37:14
והא תני רבי אושעיא חול התם במתונא תנא דידן נמי ליתני ולוקמיה במתונא הא תנא ליה אמת המים
whether moist or dry. Why is it that here flint stones are included in the list and not sand, and there sand is included and not flint stones? — R. Joseph answered: Because it is not usual to keep food warm in flint stones. Said Abaye to him: And is it usual to keep food warm in woollen fleeces and strips of purple wool? And yet [these are mentioned in] a Baraitha which says: 'Food may be kept warm in woollen fleeces and strips of purple wool and fluff, but these things must not be carried on Sabbath.' No, said Abaye. The truth is that, his neighbour telleth concerning him.<span class="x" onmousemove="('comment',' Job XXXVI 33; [H] E. V. 'noise' is rendered here 'friend', companion'; i.e. one passage elucidates the other. ');"><sup>11</sup></span>