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tirmidhi:2637 – Qutaybah > Mālik b. Anas > ʿAbdullāh b. Dīnār > Ibn ʿUmar

Ibn 'Umar narrated that the Prophet ﷺ said: "Whoever says to his brother 'disbeliever' then it will have settled upon one of them." Sahih  

الترمذي:٢٦٣٧ – حَدَّثَنَا قُتَيْبَةُ عَنْ مَالِكِ بْنِ أَنَسٍ عَنْ عَبْدِ اللَّهِ بْنِ دِينَارٍ عَنِ ابْنِ عُمَرَ

عَنِ النَّبِيِّ ﷺ قَالَ أَيُّمَا رَجُلٍ قَالَ لأَخِيهِ كَافِرٌ فَقَدْ بَاءَ بِهِ أَحَدُهُمَا هَذَا حَدِيثٌ حَسَنٌ صَحِيحٌ غَرِيبٌ وَمَعْنَى قَوْلِهِ بَاءَ يَعْنِي أَقَرَّ  

See similar narrations below:

Collected by Bukhārī, Aḥmad, Ibn Ḥibbān

You would risk becoming a disbeliever if you call another Muslim a kāfir (disbeliever)

bukhari:6104 – Ismāʿīl > Mālik > ʿAbdullāh b. Dīnār > ʿAbdullāh b. ʿUmar

The Messenger of Allah ﷺ said, "If anyone says to his brother O kāfir (disbeliever)! Then surely, one of them such."  

البخاري:٦١٠٤ – حَدَّثَنَا إِسْمَاعِيلُ قَالَ حَدَّثَنِي مَالِكٌ عَنْ عَبْدِ اللَّهِ بْنِ دِينَارٍ عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ ؓ

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ: «أَيُّمَا رَجُلٍ قَالَ لأَخِيهِ: يَا كَافِرُ، فَقَدْ بَاءَ بِهَا أَحَدُهُمَا۔»  

ahmad:5933 – Isḥāq > Mālik > ʿAbdullāh b. Dīnār > Ibn ʿUmar

Messenger of Allah ﷺ said, 'If anyone says to his brother, 'O misbeliever! Then surely, one of them such." (Using translation from Bukhārī 6104)  

أحمد:٥٩٣٣ – حَدَّثَنَا إِسْحَاقُ أَخْبَرَنَا مَالِكٌ عَنْ عَبْدِ اللهِ بْنِ دِينَارٍ عَنِ ابْنِ عُمَرَ قَالَ

قَالَ رَسُولُ اللهِ ﷺ أَيُّمَا رَجُلٍ قَالَ لِأَخِيهِ يَا كَافِرُ فَقَدْ بَاءَ بِهَا أَحَدُهُمَا  

ahmad:5259 – Wakīʿ > Sufyān > ʿAbdullāh b. Dīnār > Ibn ʿUmar

[Machine] The Messenger of Allah ﷺ said, "Whoever says to his brother 'O disbeliever,' then surely one of them has earned it."  

أحمد:٥٢٥٩ – حَدَّثَنَا وَكِيعٌ عَنْ سُفْيَانَ عَنْ عَبْدِ اللهِ بْنِ دِينَارٍ عَنِ ابْنِ عُمَرَ قَالَ

قَالَ رَسُولُ اللهِ ﷺ أَيُّمَا امْرِئٍ قَالَ لِأَخِيهِ يَا كَافِرُ فَقَدْ بَاءَ بِهَا أَحَدُهُمَا  

ibnhibban:249 – al-Ḥusayn b. Idrīs al-Anṣārī > Aḥmad b. Abū Bakr > Mālik > ʿAbdullāh b. Dīnār > Ibn ʿUmar

Messenger of Allah ﷺ said, 'If anyone says to his brother, 'O misbeliever! Then surely, one of them such." (Using translation from Bukhārī 6104)  

ابن حبّان:٢٤٩ – أَخْبَرَنَا الْحُسَيْنُ بْنُ إِدْرِيسَ الْأَنْصَارِيُّ قَالَ حَدَّثَنَا أَحْمَدُ بْنُ أَبِي بَكْرٍ عَنْ مَالِكٍ عَنْ عَبْدِ اللَّهِ بْنِ دِينَارٍ عَنِ ابْنِ عُمَرَ

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ «أَيُّمَا رَجُلٌ قَالَ لِأَخِيهِ كَافِرٌ فَقَدْ بَاءَ بِهِ أَحَدُهُمَا»