حَدَّثَنَا يَعْلَى بْنُ عُبَيْدٍ حَدَّثَنَا فُضَيْلٌ يَعْنِي ابْنَ غَزْوَانَ عَنْ نَافِعٍ عَنِ ابْنِ عُمَرَ قَالَ قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ أَيُّمَا رَجُلٍ كَفَّرَ رَجُلًا فَإِنْ كَانَ كَمَا قَالَ وَإِلَّا فَقَدْ بَاءَ بِالْكُفْرِ
مسند امام احمد بن حنبل · Hadith 4745
It was narrated that Ibn 'Umar said: The Messenger of Allah (PBUH) said: "Any man who calls another a kaafir, if he is as he says he is (then that is the end of the matter), otherwise it applies to him."
Sanad
حَدَّثَنَا يَعْلَى بْنُ عُبَيْدٍ حَدَّثَنَا فُضَيْلٌ يَعْنِي ابْنَ غَزْوَانَ عَنْ نَافِعٍ عَنِ ابْنِ عُمَرَ