It is narrated from Sayyiduna Sa'd bin Abi Waqqas (may Allah be pleased with him) that he said: The Messenger of Allah (peace and blessings be upon him) said: "Killing a Muslim is disbelief, and reviling him is wickedness (fusuq), and it is not lawful for a Muslim to cut off his brother for more than three days."
Hadith Referenceالمعجم الكبير للطبراني / العشرة / 322
It is narrated from Sayyiduna Sa'd (may Allah be pleased with him) that he said: I heard the Messenger of Allah (peace and blessings be upon him) say: "Reviling a Muslim is wickedness (fusuq), and fighting him is disbelief."
Hadith Referenceالمعجم الكبير للطبراني / العشرة / 323
Amir bin Sa'd narrates from his father, Sayyiduna Sa'd (may Allah be pleased with him), who said: A Bedouin came to the Prophet (peace and blessings be upon him) and said: My father used to maintain ties of kinship and did such-and-such good deeds, so where is he? He (peace and blessings be upon him) said: In Hellfire. The Bedouin felt hurt by this and said: O Messenger of Allah! Then where is your father? He (peace and blessings be upon him) said: "Wherever you pass by the grave of a disbeliever, give him the tidings of Hellfire." The narrator said: After that the Bedouin became Muslim and said: The Messenger of Allah (peace and blessings be upon him) has put me to hardship; whenever I pass by the grave of a disbeliever, I give him the tidings of Hellfire.
Hadith Referenceالمعجم الكبير للطبراني / العشرة / 324