Sayyiduna Abdullah bin Amr (may Allah be pleased with him) narrates: The Prophet Muhammad (peace and blessings be upon him) said: "Whoever calls his brother a disbeliever, then this disbelief returns to one of the two. If the person is as he has been described, then (it is as he said); otherwise, this (disbelief) will return to the one who said it."
Hadith Referenceصحیح ابن حبان / كتاب الإيمان / 250
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيصحيح - «الصحيحة» (2891): م.فضيلة الشيخ العلّامة شُعيب الأرناؤوطإسناده صحيح على شرط مسلم.