James 3:1-12 (KJV)
My brethren, be not many masters, knowing that we shall receive the greater condemnation. For in many things we offend all. If any man offend not in word, the same is a perfect man, and able also to bridle the whole body. Behold, we put bits in the horses' mouths, that they may obey us; and we turn about their whole body. Behold also the ships, which though they be so great, and are driven of fierce winds, yet are they turned about with a very small helm, whithersoever the governor listeth. Even so the tongue is a little member, and boasteth great things. Behold, how great a matter a little fire kindleth! And the tongue is a fire, a world of iniquity: so is the tongue among our members, that it defileth the whole body, and setteth on fire the course of nature; and it is set on fire of hell. For every kind of beasts, and of birds, and of serpents, and of things in the sea, is tamed, and hath been tamed of mankind: But the tongue can no man tame; it is an unruly evil, full of deadly poison. Therewith bless we God, even the Father; and therewith curse we men, which are made after the similitude of God. Out of the same mouth proceedeth blessing and cursing. My brethren, these things ought not so to be. Doth a fountain send forth at the same place sweet water and bitter? Can the fig tree, my brethren, bear olive berries? Either a vine, figs? So can no fountain both yield salt water and fresh.
Proverbs 12:18 (ESV)
There is one whose rash words are like sword thrusts, but the tongue of the wise brings healing.
Proverbs 25:11 (ESV)
A word fitly spoken is like apples of gold in a setting of silver.