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

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

Any person who called his brother: "O unbeliever" (has in fact done an act by which this unbelief) would return to one of them. If it were so, as he asserted (then the unbelief of man was confirmed but if it was not true), then it returned to him (to the man who labeled it on his brother Muslim).  

قَالَ رَسُولُ اللَّهِ ﷺ أَيُّمَا امْرِئٍ قَالَ لأَخِيهِ يَا كَافِرُ فَقَدْ بَاءَ بِهَا أَحَدُهُمَا إِنْ كَانَ كَمَا قَالَ وَإِلاَّ رَجَعَتْ عَلَيْهِ  

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

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

Malik related to me from Abdullah ibn Dinar from Abdullah ibn Umar that the Messenger of Allah ﷺ said, "If a man says to his muslim brother, 'O kafir!' it is true about one of them."  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ مَنْ قَالَ لأَخِيهِ يَا كَافِرُ فَقَدْ بَاءَ بِهَا أَحَدُهُمَا  

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

عَنِ النَّبِيِّ ﷺ قَالَ مَنْ كَفَّرَ أَخَاهُ فَقَدْ بَاءَ بِهَا أَحَدُهُمَا  

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

قَالَ رَسُولُ اللهِ ﷺ مَنْ قَالَ لِأَخِيهِ يَا كَافِرُ فَقَدْ بَاءَ بِهَا أَحَدُهُمَا  

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

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

[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.  

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

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

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

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

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

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

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

Any person who called his brother: "O unbeliever" (has in fact done an act by which this unbelief) would return to one of them. If it were so, as he asserted (then the unbelief of man was confirmed but if it was not true), then it returned to him (to the man who labeled it on his brother Muslim). (Using translation from Muslim 60b)  

قَالَ رَسُولُ اللَّهِ ﷺ «أَيُّمَا امْرِئٍ قَالَ لِأَخِيهِ كَافِرٌ فَقَدْ بَاءَ بِهِ أَحَدُهُمَا إِنْ كَانَ كَمَا قَالَ وَإِلَّا رَجَعَتْ عَلَيْهِ»  

"أيُّمَا امْرِئ قَال لأخيه "كَافِرٌ" فَقَدْ بَاءَ بها أحَدُهُمَا إِنْ كَانَ كمَا قَال، وَإلا رَجَعتْ عَلَيهِ ".  

"مَنْ كفَّرَ أَخاهُ فقدْ بَاءَ به أَحَدُهُمَا".  

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

"إذا أكْفَرَ الرَّجُلُ أخاهُ فَقَدْ باءَ بِها أَحَدُهما" .