Sayyiduna Abu Hurairah (may Allah be pleased with him) narrates that the Noble Prophet (peace and blessings be upon him) said: "Do you know what backbiting is?" The people replied: "Allah and His Messenger know best." The Noble Prophet (peace and blessings be upon him) said: "It is mentioning about your brother something which he dislikes." A questioner asked: "What is your opinion if that thing is actually present in my brother which I have mentioned?" The Noble Prophet (peace and blessings be upon him) said: "If that thing is present in him which you have mentioned, then you have backbitten him, and if it is not in him, then you have slandered him."
Hadith Referenceصحیح ابن حبان / كتاب الحظر والإباحة / 5759
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيصحيح - انظر ما قبله.فضيلة الشيخ العلّامة شُعيب الأرناؤوطإسناده صحيح على شرط مسلم