Reference for Moed Katan 42:20
איבעיא להו
Our Rabbis taught: A mourner who arrives home during the first three days from a place in the near vicinity, counts [his days of mourning] with them.<span class="x" onmousemove="('comment',' With the other members of the family.');"><sup>26</sup></span> If he came home from a distance, he counts on his own. Thereafter even if he came home from a place in the vicinity he counts on his own. R'Simeon says: Even if he came home on the seventh day from a place in the vicinity, he counts with them.<span class="x" onmousemove="('comment',' With the other members of the family.');"><sup>26</sup></span> The Master said: 'During the first three days from a place in the vicinity, he counts with them'. R'Hiyya b Abba, as citing R'Johanan said that this is done only where the chief person of the household was at home. The following question was then raised: