ייִדישער מחשבֿה אויף תהילים 15:3

Likewise, when one habituates his tongue to speak good, it will become part of his nature, as written "He who does not habituate/slander with his tongue" (Tehilim 15:3), and it says regarding the opposite of this "Your tongue devises mischief, [as a sharpened razor, working deceit], You love all devouring words" (Tehilim 52:4), and "What does it give to you, what does it increase unto you, O deceitful tongue?" (Tehilim 120:3), and "The words of a wise man's mouth are gracious; but the lips of a fool will swallow him up" (Koheles 10:12)

You should know that all the Duties of the Heart and all disciplines of the soul, whether positive or negative, fall within these ten roots which I have composed in this book, just like many of the commandments fall under the precepts of "love your fellow as yourself" (Levit. 19:18), and under "he did no evil to his fellow" (Ps. 15:3), and under "turn from evil and do good" (Ps. 34:15).