Al-Adab Al-Mufrad, Book 24, Hadith 17 (In-Book) ⓘ
'Abdullah said, "There is a veil from Allah Almighty between every two Muslims. When one of them says ugly words to his companion, he has rent the veil of Allah. When one of them tells the other, 'You are an unbeliever,' then one of them has disbelieved."
حَدَّثَنَا خَلاَّدُ بْنُ يَحْيَى، قَالَ: حَدَّثَنَا سُفْيَانُ، عَنْ يَزِيدَ بْنِ أَبِي زِيَادٍ، عَنْ عَمْرِو بْنِ سَلَمَةَ، عَنْ عَبْدِ اللهِ قَالَ: مَا مِنْ مُسْلِمَيْنِ إِلاَّ بَيْنَهُمَا مِنَ اللهِ عَزَّ وَجَلَّ سِتْرٌ، فَإِذَا قَالَ أَحَدُهُمَا لِصَاحِبِهِ كَلِمَةَ هَجْرٍ فَقَدْ خَرَقَ سِتْرَ اللهِ، وَإِذَا قَالَ أَحَدُهُمَا لِلْآخَرِ: أَنْتَ كَافِرٌ، فَقَدْ كَفَرَ أَحَدُهُمَا.
also referenced under: Hadith 435 (Reference) & Book 24, Hadith 435 (English)