Musar for Shabbat 193:17
אמר ליה רב חסדא לרב המנונא ואמרי לה רב המנונא לרב חסדא מנא הא מילתא דאמור רבנן כל פחות מג' כלבוד דמי אמר ליה לפי שאי אפשר לה לרשות הרבים שתילקט במלקט וברהיטני
R. Hisda asked R. Hamnuna-others state, R. Hamnuna asked R. Hisda-How do we know this principle which the Rabbis stated, viz.: Whatever is [separated by] less than three [handbreadths] is as joined?<span class="x" onmousemove="('comment',' Labud, v, Glos, ');"><sup>29</sup></span> Said he to him, Because it is impossible for the street to be trimmed with a plane and shears.<span class="x" onmousemove="('comment',' The ground cannot be perfectly levelled, and it must contain bumps of that height. Therefore everything within three handbreadths is regarded as joined to the ground. ');"><sup>30</sup></span>