This was told to Saul A Kassin from his cousin Aaron Kassin, whom the man in the story told it directly to Aaron.
A man was praying in the same Minyan as Rabbi Yaakov Kassin. Rabbi Yaakov Kassin walked by him and told him to take off his Tefillin. The man was shocked, surprised, and of course nervous, but did as the Rabbi told him. Rabbi Yaakov Kassin asked the Shamosh of Shaare Zion - Shalom Ozery to bring him a knife. Rabbi Yaakov Kassin using the knife opened the man’s Tefillin. When it was opened, they found nothing inside the Tefillin.