حَدَّثَنَا عَبْد اللَّهِ قَالَ وَجَدْتُ فِي كِتَابِ أَبِي حَدَّثَنَا حَجَّاجٌ أَخْبَرَنَا شُعْبَةُ عَنْ عَبْدِ اللَّهِ بْنِ دِينَارٍ عَنِ ابْنِ عُمَرَ عَنْ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ أَنَّهُ قَالَ إِذَا قَالَ الرَّجُلُ لِأَخِيهِ أَنْتَ كَافِرٌ أَوْ يَا كَافِرُ فَقَدْ بَاءَ بِهَا أَحَدُهُمَا
مسند امام احمد بن حنبل · Hadith 5077
It was narrated from 'Abdullah ibn Dinar, from Ibn Umar, from the Prophet (PBUH) that he said: "If a man says to his brother, O kaafir, it will (inevitably) apply to one of them."
Translated by Dr. Muhammad Muhsin Khan · Reviewed by Islamic University of Madinah · Published March 2020