Mystical Significance of Hair

 https://www.chabad.org/kabbalah/article_cdo/aid/380719/jewish/Mystical-Significance-of-Hair-Part-1.htm

Kabbalistic and Chasidic practice don't avail themselves of these lenient halachic opinions (Shulchan Aruch, Yoreh Deah 181:3 see Veyesh Osrim & 181:11 and the Arizal quoted above), and so Kabbalistic principles differs in the following ways:

1. A man does not shave, nor permit others to shave, his beard at all, with the exception of the mustache that overlaps the top lip, and this is only to be trimmed to above the lip with a scissors. (Writings of the Ari, Taamei Hamitzvot, parashat Kedoshim)