We live on the battlefield of the war of words. Let us recognize their power to create and to destroy, choosing, by the grace of God, to use their power for healing, peacemaking, truth telling, and inspiring the common good. – Lisa Degrenia

From the Book of Proverbs
Lying lips conceal hatred, and whoever utters slander is a fool.
When words are many, transgression is not lacking,
But the prudent are restrained in speech.
The tongue of the righteous is choice silver; the mind of the wicked is of little worth.
The lips of the righteous feed many, but fools die for lack of sense. (Proverbs 10:18-21)

Rash words are like sword thrusts, but the tongue of the wise brings healing.
Truthful lips endure forever, but a lying tongue lasts only a moment.
Deceit is in the mind of those who plan evil,
But those who counsel peace have joy. (Proverbs 12:18-20)

From the fruit of their words good persons eat good things,
But the desire of the treacherous is for wrongdoing.
Those who guard their mouths preserve their lives;
Those who open wide their lips come to ruin. (Proverbs 13:2-3)

A soft answer turns away wrath, but a harsh word stirs up anger.
The tongue of the wise dispenses knowledge, but the mouths of fools pour out folly.
A gentle tongue is a tree of life, but perverseness in it breaks the spirit. (Proverbs 15:1-2, 4)

From the fruit of the mouth one’s stomach is satisfied;
The yield of the lips brings satisfaction.
Death and life are in the power of the tongue,
And those who love it will eat its fruits. (Proverbs 18:20-21)

It can sway men to violence, or it can move them to the noblest actions. It can instruct the ignorant, encourage the dejected, comfort the sorrowing, and soothe the dying. Or it can crush the human spirit, destroy reputations, spread distrust and hate, and bring nations to the brink of war.
– Curtis Vaughan in James, a study guide 

From the Book of Psalms
Help, O Lord, for there is no longer anyone who is godly;
The faithful have disappeared from humankind.
They utter lies to each other; with flattering lips and a double heart they speak.
May the Lord cut off all flattering lips, the tongue that makes great boasts,
Those who say, “With our tongues we will prevail;
Our lips are our own — who is our master?” (Psalm 12:1-4)

Come, O children, listen to me; I will teach you the fear of the Lord.
Which of you desires life, and covets many days to enjoy good?
Keep your tongue from evil, and your lips from speaking deceit.
Depart from evil, and do good; seek peace, and pursue it. (Psalm 34:11-14)

Why do you boast, O mighty one, of mischief done against the godly?
All day long you are plotting destruction.
Your tongue is like a sharp razor, you worker of treachery.
You love evil more than good, and lying more than speaking the truth.
You love all words that devour, O deceitful tongue. (Psalm 52:1-4)

My companion laid hands on a friend and violated a covenant with me
With speech smoother than butter, but with a heart set on war;
With words that were softer than oil, but in fact were drawn swords.
(Psalm 55:20-21)

Hide me from the secret plots of the wicked,
From the scheming of evildoers, who whet their tongues like swords,
Who aim bitter words like arrows, shooting from ambush at the blameless;
They shoot suddenly and without fear.
They hold fast to their evil purpose;
They talk of laying snares secretly, thinking, “Who can see us?
Who can search out our crimes? We have thought out a cunningly conceived plot.”
For the human heart and mind are deep.
But God will shoot his arrow at them; they will be wounded suddenly.
Because of their tongue he will bring them to ruin;
All who see them will shake with horror. (Psalm 64:2-8)

Deliver me, O Lord, from evildoers;
Protect me from those who are violent,
Who plan evil things in their minds and stir up wars continually.
They make their tongue sharp as a snake’s,
And under their lips is the venom of vipers. (Psalm 140:1-3)

Every idle word we utter betrays our lack of respect for our neighbor, and shows that we place ourselves on a pinnacle above him and value our own lives higher than his. The angry word is a blow struck at our brother, a stab at his heart; it seeks to hit, to hurt and to destroy. A deliberate insult is even worse, for we are then openly disgracing our brother in the eyes of the world, and causing others to despise him. With our hearts burning with hatred, we seek to annihilate his moral and material existence. We are passing judgment on him, and that is murder. And the murderer himself will be judged. – Dietrich Bonhoeffer in The Cost of Discipleship

All five chapters in the book of James refer to the power of words.
Chapter 1
You must understand this, my beloved:let everyone be quick to listen, slow to speak, slow to anger (1:19)
If any think they are religious, and do not bridle their tongues but deceive their hearts, their religion is worthless. (1:26)

Chapter 2
So speak and so act as those who are to be judged by the law of liberty. (2:12)

Chapter 3
Not many of you should become teachers, my brothers and sisters, for you know that we who teach will be judged with greater strictness. For all of us make many mistakes. Anyone who makes no mistakes in speaking is perfect, able to keep the whole body in check with a bridle. If we put bits into the mouths of horses to make them obey us, we guide their whole bodies. Or look at ships: though they are so large that it takes strong winds to drive them, yet they are guided by a very small rudder wherever the will of the pilot directs.

So also the tongue is a small member, yet it boasts of great exploits. How great a forest is set ablaze by a small fire! And the tongue is a fire. The tongue is placed among our members as a world of iniquity; it stains the whole body, sets on fire the cycle of nature, and is itself set on fire by hell.

For every species of beast and bird, of reptile and sea creature, can be tamed and has been tamed by the human species, but no one can tame the tongue— a restless evil, full of deadly poison. With it we bless the Lord and Father, and with it we curse those who are made in the likeness of God. From the same mouth come blessing and cursing. My brothers and sisters, this ought not to be so. Does a spring pour forth from the same opening both fresh and brackish water? Can a fig tree, my brothers and sisters, yield olives, or a grapevine figs? No more can salt water yield fresh. (3:1-12)

Chapter 4
Do not speak evil against one another, brothers and sisters. Whoever speaks evil against another or judges another, speaks evil against the law and judges the law; but if you judge the law, you are not a doer of the law but a judge. There is one lawgiver and judge who is able to save and to destroy. So who, then, are you to judge your neighbor?(4:11-12)

Chapter 5
Above all, my beloved, do not swear, either by heaven or by earth or by any other oath, but let your “Yes” be yes and your “No” be no, so that you may not fall under condemnation. (5:12)