Likutei Moharan 27:3/ליקוטי מוהר"ן כ״ז:ג׳

13. And a majestic countenance corresponds to Torah expositions. For the Torah is expounded by Thirteen Principles, which are drawn from the Thirteen Rectifications of the ZaKaN (Beard)—the aspect of a majestic countenance. As it is written (Leviticus 19:32), “Show reverence for the presence of the ZaKeN (elder).”