'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."
Source: sunnah.com
Other translations
Arabic
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.
sunnah.com