Sahih Muslim 60
Sahih Muslim · Book 1: The Book of Faith · Clarifying the condition of faith for one who says to his muslim brother: "O Kafir (Disbeliever)."
It is reported on the authority of Ibn 'Umar that the Messenger of Allah (may peace and blessings be upon him) said:
وَحَدَّثَنَا يَحْيَى بْنُ يَحْيَى التَّمِيمِيُّ ، وَيَحْيَى بْنُ أَيُّوبَ ، وَقُتَيْبَةُ بْنُ سَعِيدٍ ، وَعَلِيُّ بْنُ حُجْرٍ ، جَمِيعًا عَنْ إِسْمَاعِيلَ بْنِ جَعْفَرٍ ، قَالَ يَحْيَى بْنُ يَحْيَى أَخْبَرَنَا إِسْمَاعِيلُ بْنُ جَعْفَرٍ، عَنْ عَبْدِ اللَّهِ بْنِ دِينَارٍ ، أَنَّهُ سَمِعَ ابْنَ عُمَرَ ، يَقُولُ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " أَيُّمَا امْرِئٍ قَالَ لأَخِيهِ يَا كَافِرُ . فَقَدْ بَاءَ بِهَا أَحَدُهُمَا إِنْ كَانَ كَمَا قَالَ وَإِلاَّ رَجَعَتْ عَلَيْهِ " .
Any person who called his brother: "O unbeliever" (has in fact done an act by which this unbelief) would return to one of them. If it were so, as he asserted (then the unbelief of man was confirmed but if it was not true), then it returned to him (to the man who labeled it on his brother Muslim).