The Messenger of Allah ﷺ said, "When a man says to his companion, 'You disbeliever,' it falls upon one of them. If the one to whom it was said is indeed a disbeliever, then he is so. Otherwise, it returns upon the one who said it."
حدثنا عفان حدثنا صخر يعني ابن جويرية حدثنا نافع أن
أن رسول الله ﷺ قال إذا قال الرجل لصاحبه يا كافر فإنها تجب على أحدهما فإن كان الذي قيل له كافر فهو كافر وإلا رجع إليه ما قال
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Its isnad is sahih. It is a longer version of 5260.As for the word "kafir" (unbeliever) mentioned there, it is written this way in the three manuscript originals, without an alif, and it is in the accusative case, being the predicate of kana. So it was written according to the dialect of those who pause on the accusative with a sukoon, writing it without an alif.See our commentary on Risalah of Shafii, paragraph 198, and the paragraphs we referred to in its indexes (page 261, number 28).