Hadith 4816
وَعَنْ أَبِي ذَرٍّ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «لَا يَرْمِي رَجُلٌ رَجُلًا بِالْفُسُوقِ وَلَا يَرْمِيهِ بِالْكُفْرِ إِلَّا ارْتَدَّتْ عَلَيْهِ إِنْ لَمْ يَكُنْ صَاحِبُهُ كَذَلِكَ» رَوَاهُ البُخَارِيّ
Abu Dharr (may Allah be pleased with him) reports that the Messenger of Allah (peace and blessings be upon him) said: "If a person calls another person a fasiq (evildoer) or a kafir (disbeliever), and if the one who is being called is not such, then that (statement) returns upon the one who said it." Narrated by al-Bukhari.