Reference for Eruvin 193:9
אי מתרי תלתא גברי זבין כל חד וחד ליבעי בדיקה לעולם מחד גברא זבין ובעינן דמיתמחי בשל יד ובשל ראש
ruled: If a man buys a supply of' tefillin<span class="x" onmousemove="('comment',' For trading purposes.');"><sup>20</sup></span> from a non-expert he must examine two tefillin of the hand and one of the head, or two of the head and one of the hand.<span class="x" onmousemove="('comment',' If the three tefillin are found on examination to be properly written and prepared the seller is presumed to be all expert and the remainder of the supply may be regarded as valid tefillin.');"><sup>21</sup></span> But, whatever your explanation may be, a difficulty remains:<span class="x" onmousemove="('comment',' Lit., 'what is your desire?'');"><sup>22</sup></span>