והיה לך לאות על ידיך ולזכון בין עיניך (בא יג:ט)

Reb Nachman Daks z’’l was stringent about not speaking while wearing tefillin. When he was bar mitzvah, Maran Rebbe Yissocher Dov of Belz told him, “If you want to merit a long life, then don’t talk while wearing tefillin.”

Reb Nachman merited to live long years.  And when he heard other bochurim say they put on tefillin by Maran, he would say that it couldn’t be because they spoke while wearing tefillin.