"مَنْ كفَّرَ أَخاهُ فقدْ بَاءَ به أَحَدُهُمَا".
You would risk becoming a disbeliever if you call another Muslim a kāfir (disbeliever)
The Messenger of Allah ﷺ said, "If anyone says to his brother O kāfir (disbeliever)! Then surely, one of them such."
أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ: «أَيُّمَا رَجُلٍ قَالَ لأَخِيهِ: يَا كَافِرُ، فَقَدْ بَاءَ بِهَا أَحَدُهُمَا۔»
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
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] 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»
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" .