Abu Hurayrah (may Allah be pleased with him) says that the Prophet (peace and blessings be upon him) said: "The believer is a mirror and a brother to the believer; when he meets him he protects him from his faults, and he guards him behind his back."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16681