Al-Mundhiri recorded the following chapter in his book Al-Targhib wal-Tarhib:

الترهيب من قوله لمسلم يا كافر

Discouragement from declaring a Muslim to be an unbeliever.

Source: al-Targhīb wal-Tarhīb 3/463

Ibn Umar reported: The Messenger of Allah, peace and blessings be upon him, said:

أَيُّمَا امْرِئٍ قَالَ لِأَخِيهِ يَا كَافِرُ فَقَدْ بَاءَ بِهَا أَحَدُهُمَا إِنْ كَانَ كَمَا قَالَ وَإِلَّا رَجَعَتْ عَلَيْهِ

When a man calls his brother an unbeliever, it turns against at least to one of them. Either the accused is as he claimed, or else the charge will turn against him.

Source: Ṣaḥīḥ al-Bukhārī 5753, Grade: Muttafaqun Alayhi

Abu Dharr reported: The Prophet, peace and blessings be upon him, said:

لَا يَرْمِي رَجُلٌ رَجُلًا بِالْفُسُوقِ وَلَا يَرْمِيهِ بِالْكُفْرِ إِلَّا ارْتَدَّتْ عَلَيْهِ إِنْ لَمْ يَكُنْ صَاحِبُهُ كَذَلِكَ

A man does not accuse another man of wickedness or unbelief except that it will be turned against him, if his companion is innocent.

Source: Ṣaḥīḥ al-Bukhārī 5698, Grade: Sahih

Abu Huraira reported: The Messenger of Allah, peace and blessings be upon him, said:

إِذَا قَالَ الرَّجُلُ لِأَخِيهِ يَا كَافِرُ فَقَدْ بَاءَ بِهِ أَحَدُهُمَا

When a man accuses his brother of being an unbeliever, then it will be turned against at least one of them.

Source: Ṣaḥīḥ al-Bukhārī 5752, Grade: Sahih

Thabit ibn al-Dahhak reported: The Prophet, peace and blessings be upon him, said:

 لَعْنُ الْمُؤْمِنِ كَقَتْلِهِ وَمَنْ رَمَى مُؤْمِنًا بِكُفْرٍ فَهُوَ كَقَتْلِهِ

Cursing a believer is like killing him, and accusing a believer of unbelief is like killing him.

Source: Ṣaḥīḥ al-Bukhārī 5754, Grade: Muttafaqun Alayhi

‘Imran ibn Husayn reported: The Messenger of Allah, peace and blessings be upon him, said:

إِذَا قَالَ الرَّجُلُ لِأَخِيهِ يَا كَافِرُ فَهُوَ كَقَتْلِهِ وَلَعْنُ الْمُؤْمِنِ كَقَتْلِهِ

If a man declares his brother to be an unbeliever, it is as if he has killed him. Cursing a believer is like killing him.

Source: Musnad al-Bazzār 3158, Grade: Sahih