صحيح الادب المفرد
Sahih al-Adab al-Mufrad
Part Two: Ahadith 250 to 499
باب من قال لأخيه : يا كافر
Chapter: Saying to one’s brother:“O disbeliever”
Hadith Number: 340
340/439 عن عبد الله بن عمر؛ ان رسول الله صلى الله عليه وسلم قال:" ايما رجل قال لاخيه: كافر، فقد باء بها احد احدهما".
It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that indeed the Messenger of Allah (peace and blessings be upon him) said: "Whoever calls his brother a disbeliever, then surely one of them will return with disbelief."
تخریج الحدیث: (صحيح)