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

[AI] From the Prophet ﷺ that he said, "When a man says to another man, 'O disbeliever,' then one of them will bear the consequences of what he said, if it is as he described. Otherwise, it will return upon the other."

عَنِ النَّبِيِّ ﷺ أَنَّهُ قَالَ إِذَا قَالَ الرَّجُلُ لِلرَّجُلِ يَا كَافِرُ فَقَدْ بَاءَ بِهِ أَحَدُهُمَا إِنْ كَانَ كَمَا قَالَ وَإِلَّا رَجَعَتْ عَلَى الْآخَرِ

[AI] 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

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»

"إذا قال الرَّجلُ لأخيه: يا كافرُ فقدْ باءَ بِهِ أحدُهُما: إنْ كانَ الذِي قيل له كافرًا (فهُوَ كافِرٌ) وإلا رجعَ إلى مَنْ قالَ".

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