Hadith 8:603
Abu Bakr narrated to us, he said: Mu'tamir ibn Sulayman narrated to us, from his father, he said: Abu 'Amr al-Shaybani narrated to us, he said: I heard 'Abdullah ibn Mas'ud saying: The Messenger of Allah (may Allah bless him and grant him peace) said: "Insulting a believer is wickedness (fisq), and fighting him is disbelief (kufr)."
حدثنا أبو بكر قال حدثنا معتمر بن سليمان عن أبيه قال حدثنا أبو عمرو الشيباني قال سمعت عبد الله بن مسعود يقول قال رسول الله صلى الله عليه وسلم سباب المؤمن فسوق وقتاله كفر