Jami` at-Tirmidhi, Hadith 2637 ⓘ
Narrated Ibn 'Umar: Ibn 'Umar narrated that the Prophet said: "Whoever says to his brother 'disbeliever' then it will have settled upon one of them." Sahih
حَدَّثَنَا قُتَيْبَةُ، عَنْ مَالِكِ بْنِ أَنَسٍ، عَنْ عَبْدِ اللَّهِ بْنِ دِينَارٍ، عَنِ ابْنِ عُمَرَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " أَيُّمَا رَجُلٍ قَالَ لأَخِيهِ كَافِرٌ فَقَدْ بَاءَ بِهِ أَحَدُهُمَا " . هَذَا حَدِيثٌ حَسَنٌ صَحِيحٌ غَرِيبٌ . وَمَعْنَى قَوْلِهِ بَاءَ يَعْنِي أَقَرَّ .
also referenced under: Book 40, Hadith 32 (In-Book) & Volume 5, Book 38, Hadith 2637 (DarusSalam)