Yadayim, Chapter 3, Mishnah 3. This tractate is devoted to the impurity of hands, a tumah instituted by the Sages rather than by the Torah. One of their enactments in this field concerns kisvei hakodesh, sacred writings. The Sages assigned such writings the level of tumah known as sheini, so that whoever takes hold of them finds his hands rendered impure. Tefillin fall under this heading, for the parchments enclosed in their compartments bear verses of Torah inscribed in holiness. Our Mishnah raises a narrow yet thoroughly practical question about them.
The Mishnah teaches: "Retzuos tefillin im hatefillin metam'os es hayadayim" - as long as the retzuos are fastened to the tefillin, they render the hands impure. Being joined to the tefillin, the straps count as one item together with them, so that grasping a strap has the identical effect on one's hands as grasping the tefillin.
"Rebbi Shimon omer, retzuos tefillin einan metam'os es hayadayim" - Rebbi Shimon disagrees. In his view the straps do not convey impurity to the hands at all. The tefillin proper, the batim with their sacred parchments, certainly do; but the straps, even while attached, remain a separate item as far as this decree is concerned, and touching them leaves the hands undisturbed.
Our Mishnah is brief, but it leaves us with a valuable window into how precisely the Sages defined the reach of what they enacted: the issue is not merely which objects possess sanctity, but exactly how far the consequences of that sanctity were legislated to travel.