Sayyiduna Mu'adh bin Jabal (may Allah be pleased with him) narrates: I was with the Prophet (peace and blessings be upon him), and the people mentioned a man in his (peace and blessings be upon him) presence and said: "How weak he is!" The Prophet (peace and blessings be upon him) said: "You have backbitten your brother." They said: "O Messenger of Allah, we only said what is in him." He (peace and blessings be upon him) said: "If you had said what is not in him, you would have slandered him."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17193