ما هي كفارة البهتان؟

If calumny (buhtan) is a lie in speech, then its expiation is sincere repentance (tawbah nasuh): remorse, desisting, and resolving never to return to it.

If it is calumny within backbiting (a lie within backbiting), then its expiation is like the expiation for backbiting: sincere repentance, in addition to seeking absolution from the wronged person if possible without causing harm. Otherwise, it is by supplicating for them, seeking forgiveness for them, and mentioning their good qualities in the same gatherings where one backbit them.