“Out of all the limbs, the easiest to move is the tongue. The most harmful of them to the Son of Adam is also the tongue. With regard to this active limb, there are two dangers- if you’re protected from at least one of them- you may not be safe from the other: the dangers of talking or remaining silent. In addition, each one of them is a greater sin than its kin. The one who is silent with the truth is a mute devil and an insubordinate to Allaah. He is a hypocrite who has no regard for his soul. And on the other hand, the one who utters falsehood is a talking sinner to Allaah.
The people of the middle are the people of the straight path. They restrain their tongues from falsehood and release them whenever they wish to harvest benefit for them in the next life. You’ll not see one of them uttering words wastefully that contain no good in them. And by no means will you see them speaking about things which may harm their souls in the hereafter. Verily (there will be) the servant who will come forth on the Day of Resurrection with good deeds paramount to mountains. But he will discover that his tongue has destroyed all of them. And another will come with evil deeds equal to the size of mountains, however, he will discover that his tongue has vanquished all of them with its frequent remembrance of Allaah and the other good mentioning connected to it.”
Ibn Al Qayyim – Among the punishments and remedies for sins.