ON THIS QUESTION there are many evidences from the Book and the Sunnah, but I will only refer to a few. Whoever has been granted success will be admonished by it, and whoever has not will not be admonished, even if there be volumes of admonition.
The purport of this chapter is that one must present to oneself the texts which we have cited about the unlawfulness of backbiting. Then one should ponder the saying of Allah 
: “Not a word does he utter but there is with him a Vigilant Watcher.” [1].
: “Not a word does he utter but there is with him a Vigilant Watcher.” [1].
Allah says: “And you deem it a light matter, though it is most grave in the sight of Allah.” [2].
says: “And you deem it a light matter, though it is most grave in the sight of Allah.” [2].
Then one should consider the hadith we have already quoted: “A person may say something that incurs the displeasure of Allah although he considers it trifling, and be hurled into the Fire of Hell because of it.” [3].One should also remember the rest of what we mentioned in the chapter on guarding the tongue and in the chapter on backbiting. He should add to this the saying: “Allah is with me, Allah beholds me, Allah watches me.”
Allāhu ma‘ī, Allāhu shāhidī, Allāhu nāẓirun ilayy.
It is related of Ḥasan al-Baṣrī, may Allah have mercy on him, that a man said to him: “You backbite me.” He replied: “Your standing with me is not such that I would give you power over my good deeds.”
We relate that Ibn al-Mubārak, may Allah have mercy on him, said: “If I were to slander anyone I would slander my parents, because they are most deserving of my good deeds.”
References
[1] Qur’ān 50:18.
[2] Qur’ān 24:15.
[3] Al-Bukhārī (6478).