Likutei Moharan, Part II 85:3/ליקוטי מוהר"ן·תנינא פ״ה:ג׳

13. And because Yaakov’s entire focus was Rachel, it is said of Rachel’s sons: of Yosef—“on the kadkod of one set apart from his brothers” (ibid. 49:26) ; and of Binyamin it is written—“and between his shoulders He dwells” (Deuteronomy 33:12). This is a clue that it is the concept of Leah, who is the first, which enables one to attain the concept of Rachel. “Kadkod” and “his shoulders” allude to the aforementioned luz .