Matthew 18:16

But if he will not hear, take with you one or two more, that "by the mouth of two or three witnesses every word may be established.'

Surrounding context

18:15 "Moreover if your brother sins against you, go and tell him his fault between you and him alone. If he hears you, you have gained your brother.

18:16 But if he will not hear, take with you one or two more, that "by the mouth of two or three witnesses every word may be established.'

18:17 And if he refuses to hear them, tell it to the church. But if he refuses even to hear the church, let him be to you like a heathen and a tax collector.

Translation and sources