Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Do you know what backbiting is?" They said: "Allah and His Messenger (peace and blessings be upon him) know best." He (peace and blessings be upon him) said: "Your mentioning your brother in a way he dislikes." It was said: "What do you think if what I say is actually present in my brother?" He said: "If what you say is present in him, you have backbitten him; and if it is not present, you have slandered him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 21163