وَعَنِ ابْنِ عُمَرَ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «أَيُّمَا رَجُلٍ قَالَ لِأَخِيهِ كَافِرُ فَقَدْ بَاءَ بِهَا أَحدهمَا» . مُتَّفق عَلَيْهِ
Ibn Umar, may Allah be pleased with him, narrates that the Messenger of Allah, peace and blessings be upon him, said: "If a person says to his (Muslim) brother: 'Disbeliever,' then surely one of them will return to disbelief (from faith)." Agreed upon.