Matthew 18:16-17

But if he does not listen, take along with you one or two others, so that EVERY WORD MAY BE CONFIRMED BY THE TESTIMONY OF TWO OR THREE WITNESSES. If he pays no attention to them [refusing to listen and obey], tell it to the church; and if he refuses to listen even to the church, let him be to you as a Gentile (unbeliever) and a tax collector. [Lev 19:17; Deut 19:15]