There is no salvation like the heart's salvation...

There is no salvation like the heart's salvation, given that all the limbs respond to its desires. If one's heart is safe, so too are the limbs, for they carry out the deeds inspired by the heart. The limbs of the corrupt become instruments through which corruption is spread.

A hadith implies that the tongue is the "interpreter of the heart." Hypocrisy is wretched because the hypocrite says with his tongue what is not in his heart. He wrongs his tongue and oppresses his heart. But if the heart is sound, the condition of the tongue follows suit. We are recommended to be upright in our speech, which is a gauge of the heart. Engaging in the regular remembrance of God (dhikr) safeguards the tongue and replaces idle talk with words and phrases that raise one in honor. The tongue is essential in developing courtesy with God, which is the whole point of existence. [Purification of the Heart]