Let not many of you be teachers, my brothers, knowing that we will receive heavier judgment.
For in many things we all stumble. If anyone doesn’t stumble in word, the same is a perfect man, able to bridle the whole body also.
Indeed, we put bits into the horses mouths so that they may obey us, and we guide their whole body.
Behold, the ships also, though they are so big and are driven by fierce winds, are yet guided by a very small rudder, wherever the pilot desires.
So the tongue is also a little member, and boasts great things. See how a small fire can spread to a large forest!
And the tongue is a fire. The world of iniquity among our members is the tongue, which defiles the whole body, and sets on fire the course of nature, and is set on fire by Gehinnom.
For every kind of animal, bird, creeping thing, and thing in the sea, is tamed, and has been tamed by mankind.
But nobody can tame the tongue. It is a restless evil, full of deadly poison.
“To what may the tongue be compared? To a dog tied with an iron chain and locked in a room within a room within a room, yet when he barks the entire populace is terrified of him. Imagine if he were loose outside! So the tongue: it is secured behind the teeth and behind the lips, yet it does no end of damage. Imagine if it were outside!”
Yalkut Shimoni cited at Chabad.org 
With it we bless our God and Father, and with it we curse men, who are made in the image of God.
Out of the same mouth comes forth blessing and cursing. My brothers, these things ought not to be so.
Does a spring send out from the same opening fresh and bitter water?
Can a fig tree, my brothers, yield olives, or a vine figs? Thus no spring yields both salt water and fresh water.
Who is wise and understanding among you? Let him show by his good conduct that his deeds are done in gentleness of wisdom.
But if you have bitter jealousy and selfish ambition in your heart, do not boast and do not lie against the truth.
This wisdom is not that which comes down from above, but is earthly, sensual, and demonic.
For where jealousy and selfish ambition are, there is confusion and every evil deed.
But the wisdom that is from above is first pure, then peaceful, gentle, reasonable, full of mercy and good fruits, without partiality, and without hypocrisy.
Now the fruit of righteousness is sown in peace by those who make peace.
1. Parshat Acharei-Kedoshim In-Depth, Chabad.org