2 Corinthians 2:10-11:

To whom ye forgive any thing, I forgive also: for if I forgave any thing, to whom I forgave it, for your sakes forgave I it in the person of Christ;
Lest Satan should get an advantage of us: for we are not ignorant of his devices.

See also:

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