Reference for Eruvin 203:9
ואמר רב יהודה אמר שמואל הלכה כרבי יהודה בנקמז וטעמא מאי אמר אביי משום דמיחזי כבונה
R'Iwya once visited Nehardea and observed that a certain man was fastening a bolt<span class="x" onmousemove="('comment',' To a door.');"><sup>28</sup></span> with a piece of reed grass.' This', he remarked: 'must not shut up'.<span class="x" onmousemove="('comment',' On the Sabbath. As reed grass is too frail to sustain the weight of a bolt it is regarded as non-existent, and the bolt must be deemed to be completely detached from the door.');"><sup>29</sup></span> R'Zera enquired: What is the ruling where the bolt was pressed into the ground?<span class="x" onmousemove="('comment',' Sc. it did not merely rest in a socket in the threshold but passed through it down into the ground under it. Is the insertion of the bolt in such a manner, it is asked, regarded as building?');"><sup>30</sup></span>