That we must depart from him who lives irregularly and contrary to discipline
Paul to the Thessalonians: “But we have commanded you, in the name of Jesus Christ, that you depart from all brethren who walk disorderly, and not according to the tradition which they have received from us.” Also in the forty-ninth Psalm: “If you saw a thief, at once you ran with him, and placed your portion with the adulterers.”]
Source: The Treatises of Cyprian (New Advent)