Likutei Moharan 2:2/ליקוטי מוהר"ן ב׳:ב׳

12. Now, this weapon must be received by means of the aspect of Yosef—i.e., guarding the brit (Covenant), as in (Psalms 45:4), “Gird your sword upon your thigh.” It is also written (Psalms 132:11, 12), “Of your offspring I will set upon your throne”—this refers to Mashiach, who is the aspect of prayer—“if your children will guard My brit”—by the means of the aspect of Yosef.

2Yosef, who guarded the brit, gained the rights of the firstborn. This corresponds to the divine service of prayer, which is an aspect of the double portion [inherited by the firstborn]. For prayer is itself twofold, as it is comprised of both praise of God and requesting one’s needs. This corresponds to “a double-edged sword in their hand” (Psalms 149:6) —i.e., two edges, a double portion. It was taken from Reuven, because he defiled his father’s bed (Genesis 49:4) ; for [the birthright] is dependent upon guarding the brit .