That he heard the Prophet ﷺ 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."
أَنَّهُ سَمِعَ النَّبِيَّ ﷺ يَقُولُ لاَ يَرْمِي رَجُلٌ رَجُلاً بِالْفُسُوقِ وَلاَ يَرْمِيهِ بِالْكُفْرِ إِلاَّ ارْتَدَّتْ عَلَيْهِ إِنْ لَمْ يَكُنْ صَاحِبُهُ كَذَلِكَ
That he heard the Prophet ﷺ 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." (Using translation from Bukhārī 6045)
أَنَّهُ سَمِعَ رَسُولَ اللهِ ﷺ يَقُولُ لَا يَرْمِي رَجُلٌرَجُلًا بِالْفِسْقِ وَلَا يَرْمِيهِ بِالْكُفْرِ إِلَّا ارْتَدَّتْ عَلَيْهِ إِنْ لَمْ يَكُنْ صَاحِبُهُ كَذَلِكَ