صحیح ابن حبان
Ibn Hibban
كتاب الإيمان
Book of Faith
باب ما جاء في صفات المؤمنين - فصل ذكر البيان بأن من أكفر إنسانا فهو كافر لا محالة-
Chapter: Qualities of the Believers – Section: Clarification that whoever declares another a disbeliever is himself a disbeliever
2 hadith
Hadith 248
Sayyiduna Abu Sa'id Khudri (may Allah be pleased with him) narrates: The Prophet Muhammad (peace and blessings be upon him) said: "Whenever a person calls another person a disbeliever, then that disbelief returns to one of the two. If the (other person) is indeed a disbeliever (then it is correct), otherwise the first person, due to calling him a disbeliever, himself becomes a disbeliever."
Hadith 249
Sayyiduna Abdullah bin Umar (may Allah be pleased with him) narrates this statement of the Noble Prophet: "Whoever calls his brother a disbeliever, then this (disbelief) returns to one of the two."