Saint 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 gained your brother. But if he does not listen, take one or two others along with you, that every word may be confirmed by the evidence of two or three witnesses. If he refuses to listen to them, tell it to the church; and if he refuses to listen even to the church, let him be to you as a Gentile and a tax collector.
Truly, 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 I say to you, if two of you agree on earth about anything they ask, it will be done for them by my Father in heaven.
For where two or three are gathered in my name, there am I in the midst of them."