(202) بَابُ سِبَابِ الْمُسْلِمِ فُسُوقٌ
'Abdullah said, "There is a veil from Allah Almighty between everytwo Muslims. When one of them says ugly words to his companion, he hasrent the veil of Allah. When one of them tells the other, 'You are an unbeliever,'then one of them has disbelieved."
Reference: Al-Adab Al-Mufrad 435
In-book reference: Book 24, Hadith 17
English translation: Book 24, Hadith 435