Backbiting

Surat Al-Ĥujurāt 49:12 O believers! Avoid most conjectures. Indeed, some conjectures are sins (liable to punishment in the Hereafter). And spy not (on someone’s unknown matters or secrets), nor backbite one another. Would any of you like to eat the flesh of his dead brother? You would hate it. And fear Allah (in all such matters). Indeed, Allah is Ever-Returning, Ever-Merciful.

Anas (May Allah be pleased with him) said: The Messenger of Allah (ﷺ) said, "During the Mi'raj (the Night of Ascension), I saw a group of people who were scratching their chests and faces with their copper nails. I asked, 'Who are these people, O Jibril?' Jibril replied: 'These are the people who ate flesh of others (by backbiting) and trampled people's honour."'

[Abu Dawud]. Riyad as-Salihin » The Book of the Prohibited actions Sunnah.com reference : Book 18, Hadith 16 Arabic/English book reference : Book 18, Hadith 1526