Sayyiduna Abu al-Darda' (may Allah be pleased with him) says that a man spoke ill of another man in the presence of the Prophet (peace and blessings be upon him), and a man defended him. So he (peace and blessings be upon him) said: "Whoever defends the honour of his Muslim brother, it will be a screen for him from Hell." [Da'if (weak)]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16684