Parsha Mitzvot-Vaetchanan-Mitzvot 420-1 – Concepts 79-80:Tefillin Upon Entering a Synagogue
The Radbaz was asked to explain the custom in his day of putting to Tefillin on at home and walking, with them on, to the synagogue.
The Radbaz explains that he has this in the name of Rabbi Simeon bar Yochai, the author of the Zohar: “On entering the synagogue one says, ‘As for me, in the abundance of Your lovingkindness will I come into Your house, I will worship toward Your holy temple in the fear of You.’ (Psalms 5:8) And fear is represented by the tefillin.
The Radbaz then states a general rule: whenever you find that the words of the Kabbalists disagree with the decision of the Talmud you must follow the Talmud and the codes. But whenever, as in this instance, there is no actual disagreement, since the matter is referred to neither in the Talmud nor in the codes, it seems right to rely on the Kabbalah. – Teshuvot Radbaz #1,111