Musar su Salmi 34:14

The fifth section: One who says to his fellow that he will benefit him and give him a gift; but as he is still speaking, he tells his heart that he will not give [it]. And it is stated (Psalms 34:14), “Guard your tongue from evil, your lips from deceitful speech.” And our Rabbis, may their memory be blessed, explained it (Bava Metzia 49a) [as saying that] one should not speak one way with the mouth and [another way] with the heart. And they also said (Yerushalmi Bava Metzia 4:2) [that] there are words that, if he goes back on them, are considered bad faith - but only if he says it with the mouth, and decides upon it with the mind. (It appears [correct] to me to explain the words, “and only (oubilvad),” as being in the place of the word, “rather.” And he means to say that the word of a man should only be in such a way that he says it with the mouth and decides upon it with the mind, which is the opposite of, one thing with the mouth and [another] with the heart.

We find that Separation is to Piety as Watchfulness is to Zeal. For the former concerns "turning from evil" (Tehilim 34:14), while the latter concerns "doing good" (ibid).

Because so many things depend upon the tongue, it is necessary to guard the tongue very much. And therefore David said, "Keep thy tongue from evil" (Ps. 34:14). And our Sages, of blessed memory, recounted a story about a certain man who cried out in the streets, "Who wants to purchase the elixir of life?" Everybody came to buy. He then took out the Book of Psalms and showed them what was written in it: "Who is the man that desireth life and loveth days, that he may see good therein? Keep thy tongue from evil" (Ps. 34:13—14). And when Rabbi Yannai saw this, he took this man into his house and he fed him and he gave him drink and money. The pupils of Rabbi Yannai came to him and said to him, "Did you not know this verse before?" And he said to them, "I did know it — but I never put it in my heart to be careful concerning this. When I used to read this verse I would go over it hastily and did not realize its full meaning. And now this man came and made it mean something to me, and from now on I shall be more careful with my tongue" (Lev. Rabbah 16:2; Shohar Tov, 52b).