Proverbs 11:17:

The merciful man doeth good to his own soul: but he that is cruel troubleth his own flesh.

See also:

Healing and Giving

God, Satan or You?

Unforgiveness Is Itself a Forgiven Sin, So How Could It Hinder My Healing?

Am I Sick Because God Has Turned Me Over to the Tormentors Because I Didn’t Forgive Someone?

Mistake: Walking out of Love