Reference for Eruvin 190:17

תנא דבי מנשה (דברים ו, ח) על ידך זו קיבורת בין עיניך זו קדקד היכא אמרי דבי רבי ינאי מקום שמוחו של תינוק רופס

was the proper [manner of wearing them] so.<span class="x" onmousemove="('comment',' As he did not say this, the question arises again: Why did R. Gamaliel allow two tefillin on the arm?');">³¹</span> - The explanation rather is this:<span class="x" onmousemove="('comment',' Lit., 'but'.');">³²</span> As R'Samuel son of R'Isaac stated: 'There is room enough on the head for laying two tefillin' so we may here also submit: There is room enough on the hand for laying two tefillin.

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