Reference for Eruvin 193:11

והתני רב כהנא בתפילין בודק שתים של יד ושל ראש הא מני רבי היא דאמר בתרי זימני הוי חזקה

The fact is that he bought them from one man, but it is necessary that his reputation shall be established in respect of those of the hand as well as those of the head. But can this be correct? purely Rabbah B'Samuel learned, 'In the case of tefillin one examines three of the hand and of the head', which means, does it not, either three of the hand or three of the head? - No, three, some of which must be of the hand and some of the head.

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