The Zohar and Halacha
The Maharshal ranked the Zohar with other midrashic literature, and when asked to rule that tefillin be put on the arm while sitting, in line with the Zohar, he adamantly refused. He cited the sages of earlier generations who had not changed their custom, and he held the same regarding tefillin on Chol Hamoed.
Source: Teshuvos Maharshal 98