Volume 8, Book 73, Number 71:
Narrated Abu Dhar:
That he heard the Prophet saying, “If somebody accuses another of Fusuq (by calling him ‘Fasiq’ i.e.
a wicked person) or accuses him of Kufr, such an accusation will revert to him (i.e. the accuser) if his
companion (the accused) is innocent.”