Proverbs 18:21:

Death and life are in the power of the tongue: and they that love it shall eat the fruit thereof.

Notes on Proverbs 18:21:

A lot depends on YOU, not on God.  What you speak can bring you life or death.  God wants you to speak things that bring life, but He won’t force you to do it.  You will not get healed or stay healed if your mouth keeps speaking death instead of life.

This verse is often preached as if the ability to speak death is a bad thing.  You can use it against yourself, and that’s a bad thing, but you can also speak death to things like cancer, and then it’s a good thing that death is in the power of your tongue!  I’ve seen terminal cancer disappear when I spoke death to it without even touching the person who had it.  It’s a wonderful thing to know your authority as a believer!  Jesus spoke death to a fig tree, so we can speak death too!

See also:

Notes on Matthew 21:18-22
Notes on James 3:2-6
Say What
Healing Does Not Require Great Faith
You Have Authority over Your Body
Quit Worrying About Your Health!
Speaking to Mountains, Trees and Diseases
We Hold the Keys
Different Ways to Get Healed
Healing by Spoken Words
Who Is in Control of Your Life?
Does Mark 11:23 Work for Unbelievers?
Objection: God Is in Control
Objection: It Is Presumption to Think That Life and Death Are in Your Power
Objection: Only Those with Gifts of Healings Receive the TEMPORARY Right to Speak Directly to an Illness
Objection: Divine Healing Is Practiced by False Cults
Objection: If Proverbs 18:21 Meant What Faith Preachers Say It Does, Old Testament People Could Have Gone Around Speaking Health to the Sick and Death to Cancer and Other Diseases Before Jesus Came
Objection: Faith Is Simply Trust in God, Not a Force That You Use to Change Things Yourself