"O assembly of those who have believed with their tongues, but into whose hearts faith has not yet reached! Do not backbite the Muslims, nor seek out their secrets! For, whoever seeks out the faults of his brother, Allah will seek out his secrets. And, whoever has his secrets sought out by Allah, Allah will disgrace him, even [if he hides] in the depths of his house. [Abu Dawud in al-Adab, 4/271, #4880]
"Beware of backbiting, for backbiting is more serious than adultery. A man may commit adultery, and drink [wine], and then repent, and Allah will forgive him. But, the backbiter will not be forgiven by Allah until his [backbited] companion forgives him." [Suyuti, Al-Jami` as-Saghir, 1/174, #2919, from Ibn Abid-Dunya, and Abush-Shaykh, Al-Tawbikh.]