Matthew 18:15-17 
"And if your brother sins, go and reprove him in private; if he listens to you, you have won your brother. [16] "But if he does not listen to you, take one or two more with you, so that by the mouth of two or three witnesses every FACT MAY BE CONFIRMED. [17] "And 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-gatherer."

(New American Standard Bible)