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.
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:6104Ismāʿīl > 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:٦١٠٤حَدَّثَنَا 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:60aAbū 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:٦٠aحَدَّثَنَا 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:6280Ibn 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:٦٢٨٠حَدَّثَنَا 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:5914Muʾammal > 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:٥٩١٤حَدَّثَنَا 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:5933Isḥāq > 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:٥٩٣٣حَدَّثَنَا 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:5077And 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:٥٠٧٧قَالَ 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:4687Yaḥyá > 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:٤٦٨٧حَدَّثَنَا يَحْيَى عَنْ سُفْيَانَ حَدَّثَنِي ابْنُ دِينَارٍ سَمِعْتُ ابْنَ عُمَرَ

عَنِ النَّبِيِّ ﷺ قَالَ إِذَا أَحَدُكُمْ قَالَ لِأَخِيهِ يَا كَافِرُ فَقَدْ بَاءَ بِهَا أَحَدُهُمَا  

ahmad:5259Wakīʿ > 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:٥٢٥٩حَدَّثَنَا 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:249al-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:٢٤٩أَخْبَرَنَا 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»