Mahmud ibn Ghaylan reported it from Waki, from Sufyan, from Zubayd, from Abu Wa’il, from Abdullah Ibn Mas’ud (RA) , saying that the Prophet (SAW) said, “To abuse a Muslim is sinful and to kill him is to disbelieve.”
[Ahmed 3647,Bukhari 48,Muslim 64,Nisai 4115,Ibn Majah 69]