Reviling a Muslim is Fusūq (deobedience) and fighting him is Kufr (disbelief)

Explanation

The Prophet (may Allah's peace and blessings be upon him) forbade the Muslim from cursing and reviling his Muslim brother, stating that this is an act of Fusūq, which means disobeying Allah and His Messenger (may Allah's peace and blessings be upon him), and fighting his Muslim brother is an act of Kufr, yet it is minor Kufr.

Benefits

• It is obligatory to respect the Muslim's honor and life.
• The grave outcome of one who reviles a Muslim unjustly, for such a person is Fāsiq (defiant sinner).
• Reviling and fighting a Muslim weakens and diminishes faith.
• Some acts are called "Kufr" although they do not reach the major Kufr that results in apostasy.
• "Kufr" here refers to minor Kufr that does not result in apostasy according to the unanimous agreement of Ahl-us-Sunnah, since Allah Almighty has established the brotherhood of faith for the believers in case of fighting and disputing, as He says: {If two groups of the believers fight one another, make peace between them} until: {The believers are but brothers.}