In כי תצא, there is a Pasuk that says, מוצא שפתיך תשמור, that one needs to guard his lips. Rabbi Soloveitchik explains this to mean that the tie that bounds the heart and tongue, must not be loosened.
The heart and tongue should go together. We must be careful how to use speech. One of the great dangers is the lure of the tongue.
We must never minimize the power of speech. One of the themes of the High Holiday season is אמת and שלום. One cannot achieve complete peace of mind if he is not truthful. Truthfulness begins with speech. If one is always careful to tell the truth, it might help to face the world without delusions and rationalizations.
Not only is it healthier to face the truth about one’s existence, but it is the key towards achieving real happiness. And this happiness begins by being truthful with the tongue. One’s heart and one’s mouth must be the same.