Is there a specific supplication for backbiting and slander other than the supplication for expiation of the مجلس (gathering)?

There is no specific supplication for backbiting and slander, and what has been narrated in some hadith that the expiation for backbiting is to seek forgiveness for the one you backbit about is not sound. Some scholars mentioned that the backbiter should praise the one he backbit about and seek forgiveness for him in proportion to the backbiting. This is the choice of Sheikh Al-Islam Ibn Taymiyyah. The supplication for the expiation of a gathering (Kaffarat al-Majlis) is not sufficient for repentance from backbiting and slander. It is obligatory for anyone present at a backbiting gathering to defend his brother's honor. If he is unable to do so, he should leave the gathering. If he does not do so, then he must repent.