James 5:15

And the prayer of faith shall save the sick, and the Lord shall raise him up; and if he has committed sins, they shall be forgiven him. 

The same faith that believes for the forgiveness of sins is the same faith that believes for healing.  The devil will try to make us doubt the word of God, and so we don’t pray for our healing.  The bible says we were healed and that God is waiting for our prayer of faith.  Then stand on the word of God, and when the devil tries to bring in doubt, remind him what the bible says.