Likutei Moharan 11:2/ליקוטי מוהר"ן י״א:ב׳

12 . However, it is impossible for a person to merit having his speech enlighten him except through glory. That is, he should see to it that the Holy One’s glory is complete; his own glory should be non-existent in relationship to the Holy One’s glory. [This is achieved] by means of humility and humbleness. For the essence of speech comes from glory, as is written (Psalms 24:10), “Who is this MeLeKh (king) of glory?”—i.e., “MaLKhut (Kingship) is mouth” (Tikkuney Zohar, Second Introduction).

2For when the Torah comes into tarnished speech—a foul mouth—not only do the words of Torah not enlighten him, but the Torah itself becomes corporal and blackened there in his mouth. As is written (Joshua 1:8), “This Book of Torah should not YaMuSh (depart) from your mouth,” corresponding to (Exodus 10:21), “darkness YaMaSh (that was tangible).” In other words, [the words of Torah] should not become corporal and blackened by your mouth. By failing to make certain that the Holy One’s glory is complete—i.e., by being haughty—a person cannot open his mouth, as in (Psalms 17:10), “Their mouth closed up, [for] they spoke haughtily.” This resembles the story of Levi bar Sisa. He was called up to the podium, but his spirit grew haughty and he was unable to speak (Yerushalmi, Yevamot chap. 12).

3As a result of haughtiness, a person is in an aspect of idolatry. Concerning the worship of idols [it is written] (Deuteronomy 7:5), “the images of their gods destroy by fire.” In connection to a shofar used for idolatry, the Talmud teaches: Anything about to be destroyed by fire is already considered destroyed and utterly reduced in size (Rosh HaShanah 28a). And, since his size has been utterly reduced, he no longer has a voice with which to speak.

4However, when a person is careful and makes certain that God’s glory is complete—he is “lowly, and in his own eyes repulsive” (Psalms 15:4) —this enables him to speak words which enlighten. As is written (Ezekiel 43:2), “And the earth shone with His glory.” [His words] enlighten him to repentance, and he can then come to deep Torah understanding.