Musnad Abu Ya'la al-Mawsili 152:51

Muhammad ibn Abi Bakr narrated to us, Muhammad ibn Dinar narrated to us, from Ibrahim al-Hajari, from Abu al-Ahwas, from Abdullah, from the Prophet (may Allah bless him and grant him peace), who said: "Insulting a Muslim is wickedness, and fighting him is disbelief. The sanctity of his wealth is like the sanctity of his blood."