Matthew 18:15-17

15 But if thy brother shall offend against thee, go, and rebuke him between thee and him alone. If he shall hear thee, thou shalt gain thy brother.
16 And if he will not hear thee, take with thee one or two more: that in the mouth of two or three witnesses every word may stand.
17 And if he will not hear them: tell the church. And if he will not hear the church, let him be to thee as the heathen and publican.