Al-Adab Al-Mufrad #442
Disparaging
The Arabic includes the chain of narration (sanad) and the collector's commentary; the English translation shows the Prophet's words only.
حَدَّثَنَا سَعِيدُ بْنُ دَاوُدَ، قَالَ: حَدَّثَنَا مَالِكٌ، أَنَّ نَافِعًا حَدَّثَهُ، أَنَّ عَبْدَ اللهِ بْنَ عُمَرَ أَخْبَرَهُ، أَنَّ رَسُولَ اللهِ صلى الله عليه وسلم قَالَ: إِذَا قَالَ لِلْآخَرِ: كَافِرٌ، فَقَدْ كَفَرَ أَحَدُهُمَا، إِنْ كَانَ الَّذِي قَالَ لَهُ كَافِرًا فَقَدْ صَدَقَ، وَإِنْ لَمْ يَكُنْ كَمَا قَالَ لَهُ فَقَدْ بَاءَ الَّذِي قَالَ لَهُ بِالْكُفْرِ.
'Abdullah ibn 'Umar reported that the Messenger of Allah, may Allah
bless him and grant him peace, said, "When someone says to another, 'Unbeliever!'
then one of them is an unbeliever. If the one to whom he says it is an
unbeliever, he has spoke the truth. If that is not the case, then the one
who said it has brought down disbelief on himself."
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About Al-Adab Al-Mufrad
This hadith is #442 from Al-Adab Al-Mufrad, in the Book of Disparaging.