Likutei Moharan 60:7/ליקוטי מוהר"ן ס׳:ז׳

17. Now, when someone works to rouse people, he must protect himself from unworthy students lest their evil attach itself to him and harm him. As our Sages, of blessed memory, said: Whoever teaches an unworthy student… (Chullin 133a) .

2Likewise, our Sages, of blessed memory, prohibited writing [the tefilin scrolls] on the skin of a non-kosher animal, as it is written (Exodus 13:9), “in order that God’s Torah may be in your mouth”—from what is permitted to your mouth (Shabbat 108a). And when one person teaches another, it is an aspect of writing, since the tongue is the aspect of “My tongue is the pen of a skillful scribe” (Psalms 45:2) ; [the teaching] is engraved and written on the student’s heart, as in (Proverbs 3:3), “write them on the tablet of your heart.” This is why it is imperative <that he take care> that his words not be written on the aspect of the skin of a non-kosher animal—i.e., a student who is unworthy.

3Even so, it is impossible for a human being to protect himself so that unworthy students do not hear him. For this, it is necessary that his teaching be “to study, teach, safeguard and laASot (practice)”—i.e., studying with his student should be as if he made his companion and as if he made the words of Torah. As our Sages, of blessed memory, said: Whoever teaches his companion’s son Torah, it is as if ASa’o (he made him)… and as if ASa’o (he made it)—the words of Torah… (Sanhedrin 99b). And when he teaches with this intention, God <Himself> protects him, keeping his words from being recorded in the memory of an unworthy student; but rather, [the student] forgets them.

4And when a person works to rouse [people] through telling stories, his conversation and telling likewise must be in the aforementioned aspect of the teaching—i.e., to study and laasot, as explained above. This is the aspect of “The conversation of Torah scholars requires study” (Sukkah 21b)—i.e., whatever is needed for the teaching is likewise needed for his conversation, which is the aspect of telling stories, as explained above.

5This is the explanation of “their leaves litruphah (for healing)” (Ezekiel 47:12). “Their leaves” is the aspect of the conversation of Torah scholars, as our Sages, of blessed memory, said <regarding the verse> “Their leaves will not wither” (Psalms 1:3)—even the conversation of Torah scholars [requires study] (Sukkah, ibid.). And this is “LiTRuPhaH”: L’haTiR PeH (to unbind the mouth)—i.e., the aspect of “from what is muTaR to your PeH (permitted to your mouth).” When his conversation is in the aspect of “the conversation of Torah scholars requires study”—i.e., “to study, teach, safeguard and practice,” as explained above—then he is protected from unworthy students. This is the aspect of “from what is permitted to your mouth”; “How do we know that we do not write [tefilin except on the skin of a kosher animal]…,” as explained above.