Caution: Translations of Quran and Ḥadīth may lead to possible misapplications and misinterpretations. This site is intended for students of sacred knowledge that are proficient in comprehending classical Arabic and have a strong foundation in Islamic sciences. Also note that religious injunctions rely on several aspects beyond what one may glean through reading individual aḥādīth.
Qurʾān Bukhārī Muslim Nasāʾī Abū Dāwūd Tirmidhī Ibn Mājah Mālik Aḥmad Dārimī Ḥākim Ibn Ḥibbān Ṭabarānī Nasāʾī's Kubrá Bayhaqī al-Luʾluʾ wa-al-Marjān Ibn Rajab's Fifty Riyāḍ al-Ṣāliḥīn Aḥmad's Zuhd Suyūṭī
suyuti:1355a –

When a man calls his brother an unbeliever, it returns (at least) to one of them. (Using translation from Muslim 60a)   

السيوطي:١٣٥٥a –

"إذا أكْفَرَ الرَّجُلُ أخاهُ فَقَدْ باءَ بِها أَحَدُهما" .  

[م] مسلم عن ابن عمر

See similar narrations below:

Collected by Bukhārī, Muslim, Aḥmad, Ibn Ḥibbān, Suyūṭī

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۔»  

muslim:60a – Abū Bakr b. Abū Shaybah > Muḥammad b. Bishr And ʿAbd Allāh b. Numayr > ʿUbaydullāh b. ʿUmar > Nāfiʿ > Ibn ʿUmar

When a man calls his brother an unbeliever, it returns (at least) to one of them.  

مسلم:٦٠a – حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ حَدَّثَنَا مُحَمَّدُ بْنُ بِشْرٍ وَعَبْدُ اللَّهِ بْنُ نُمَيْرٍ قَالاَ حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ عُمَرَ عَنْ نَافِعٍ عَنِ ابْنِ عُمَرَ

أَنَّ النَّبِيَّ ﷺ قَالَ إِذَا كَفَّرَ الرَّجُلُ أَخَاهُ فَقَدْ بَاءَ بِهَا أَحَدُهُمَا  

ahmad:6280 – Ibn Numayr And Ḥammād b. Usāmah > ʿUbaydullāh > Nāfiʿʿan Ibn ʿUmar

[Machine] "From the Prophetﷺ who said: Whoever disbelieves in his brother, one of them will bear the consequences."  

أحمد:٦٢٨٠ – حَدَّثَنَا ابْنُ نُمَيْرٍ وَحَمَّادُ بْنُ أُسَامَةَ قَالَا حَدَّثَنَا عُبَيْدُ اللهِ عَنْ نَافِعٍعَنِ ابْنِ عُمَرَ

عَنِ النَّبِيِّ ﷺ قَالَ مَنْ كَفَّرَ أَخَاهُ فَقَدْ بَاءَ بِهَا أَحَدُهُمَا  

ahmad:5914 – Muʾammal > Sufyān > ʿAbdullāh b. Dīnār > Ibn ʿUmar

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

أحمد:٥٩١٤ – حَدَّثَنَا مُؤَمَّلٌ حَدَّثَنَا سُفْيَانُ حَدَّثَنَا عَبْدُ اللهِ بْنُ دِينَارٍ سَمِعْتُ ابْنَ عُمَرَ يَقُولُ

قَالَ رَسُولُ اللهِ ﷺ مَنْ قَالَ لِأَخِيهِ 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:5077 – And Jadt Fī Kitāb Abū > Ḥajjāj > Shuʿbah > ʿAbdullāh b. Dīnār > Ibn ʿUmar

[Machine] On the authority of the Prophet ﷺ , he said: When a man says to his brother, "You are a disbeliever" or "O disbeliever," then one of them has brought it upon himself.  

أحمد:٥٠٧٧ – قَالَ وَجَدْتُ فِي كِتَابِ أَبِي حَدَّثَنَا حَجَّاجٌ أَخْبَرَنَا شُعْبَةُ عَنْ عَبْدِ اللهِ بْنِ دِينَارٍ عَنِ ابْنِ عُمَرَ

عَنِ النَّبِيِّ ﷺ أَنَّهُ قَالَ إِذَا قَالَ الرَّجُلُ لِأَخِيهِ أَنْتَ كَافِرٌ أَوْ يَا كَافِرُ فَقَدْ بَاءَ بِهَا أَحَدُهُمَا  

ahmad:4687 – Yaḥyá > Sufyān > Ibn Dīnār > Ibn ʿUmar

[Machine] The Prophet ﷺ said, "If any of you says to his brother, 'O disbeliever,' then it will surely apply to one of them."  

أحمد:٤٦٨٧ – حَدَّثَنَا يَحْيَى عَنْ سُفْيَانَ حَدَّثَنِي ابْنُ دِينَارٍ سَمِعْتُ ابْنَ عُمَرَ

عَنِ النَّبِيِّ ﷺ قَالَ إِذَا أَحَدُكُمْ 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»