“Do you know what backbiting is?”

Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: “Do you know what backbiting is?” They (the Companions) said: “Allah and His Messenger know best.” He said: “It is to say of your brother what he dislikes.” It was said: “What do you think if what I say of my brother is true?” He said: “If what you say of him is true, then you have backbitten him; if not, then you have slandered him.”  (Muslim).