Musannaf Ibn Abi Shayba 1:1429

Abu Khalid al-Ahmar narrated to us, from Muhammad ibn ‘Ajlan, from al-Harith, who said: I was holding the hand of Ibrahim, and I mentioned a man and backbited him. He said: So he said to me, "Go back and perform ablution." They used to consider this [backbiting] as *hajr* (a foul or vain speech).