In the Quran, Allah tells us
“neither backbite one another. Would one of you like to eat the flesh of his dead brother? You would hate it (so hate backbiting). And fear Allaah. Verily, Allaah is the One Who forgives and accepts repentance, Most Merciful.” [al-Hujuraat 49:12]
This video really brings the message home of what it is like when we speak ill about others behind their back.