James 3:2-6:

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.

Notes on James 3:2-6:

If this passage doesn’t prove that your tongue can control your body, I don’t know what does.  If you can control your tongue, you can control your whole body.  Note that your tongue can also defile your whole body.  Speaking health will result in health, but speaking sickness will result in sickness.  Also, speaking things you ought not to speak can result in sickness even though you are not explicitly “confessing” sickness.  If you use your tongue to gossip, criticize and backbite, your will undermine your health, “setting on fire the course of nature” in your body and defiling it.  This is a good example of why you need to know Scripture in general, not just healing Scriptures.  Failure to keep your tongue in line can keep you from getting healed.  This agrees with Proverbs 18:21 that death and life are in the power of your tongue.

See also:

You Have Authority over Your Body
Say What
Healing Does Not Require Great Faith
Who Is in Control of Your Life?
You Have Authority over Your Body
Say What
Can an Unbeliever Exercise Faith?
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: 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