John Lightfoot on Matthew 18:15

[Tell him his fault between thee and him alone.] The reason of the precept is founded in that charitable law, Leviticus 19:17; "Thou shalt not hate thy brother in thy heart; but thou shalt surely reprove him, and shalt not suffer sin in him."

Here the Talmudists speak not amiss: "The Rabbins deliver, 'Thou shalt not hate thy brother in thy heart.' Perhaps he does not beat him, he does not pull off his hair, he does not curse him: the text saith, 'in thy heart,' speaking of hatred in the heart. But whence is it proved that he that sees his brother doing some foul action is bound to reprove him? Because it is said, In reproving, thou shalt reprove. He reproves, but he heareth not: whence is it proved he is bound to a second reproof? The text saith, 'In reproving, thou shalt reprove.'" And a little after, "How long must we reprove? Rabh saith, 'Even to blows'"; that is, until he that is reproved strikes him that reproves him: "Samuel saith, 'Until he is angry.'" See also Maimonides.