1 Corinthians 9:27:

But I keep under my body, and bring it into subjection: lest that by any means, when I have preached to others, I myself should be a castaway.

See also:

Your Spirit Isn’t Sick
You Have Authority over Your Body
It Isn’t YOUR Sickness
Healing and Effort
What Romans 8:11 Really Means
Can I Believe for “Supernatural Weight Loss?”
Objection: God Is in Control
Mistake: Carnality