Matthew 18:15-20

Jesus said to his disciples: "If your brother sins against you, go and tell him his fault between you and him alone. If he listens to you, you have won over your brother.
If he does not listen, take one or two others along with you, so that 'every fact may be established on the testimony of two or three witnesses.'
If he refuses to listen to them, tell the church. If he refuses to listen even to the church, then treat him as you would a Gentile or a tax collector.
Amen, I say to you, whatever you bind on earth shall be bound in heaven, and whatever you loose on earth shall be loosed in heaven.
Again, (amen,) I say to you, if two of you agree on earth about anything for which they are to pray, it shall be granted to them by my heavenly Father.
For where two or three are gathered together in my name, there am I in the midst of them."