It is narrated from Sayyiduna Imran bin Husayn (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Cursing a believer is like killing him, and when a person says to his (believing) brother: 'O disbeliever,' then this (declaring him a disbeliever) is also like killing him."
Imam Ahmad bin Mani' recorded this hadith, and its basis is found in Sahih al-Bukhari (6105) and Sahih Muslim (110) through the narration of Sayyiduna Thabit bin al-Dahhak (may Allah be pleased with him), on account of which this hadith is sahih (authentic).
Hadith Referenceالمطالب العالية بزوائد المسانيد الثمانية / كتاب الأدب / 2715
Hadith Gradingمحدثین:إسناده صحيح
Hadith Takhrij«حکم الحدیث المطالب العالیہ محققاً تنسیق د. سعد بن ناصر بن عبد العزيز الشَّثري- مکتبہ شاملہ :صحيح. » «أخرجه مسند البزار برقم: 3518، وأخرجه المعجم الكبير للطبراني برقم: 463»