 However, Shia Muslims accuse him of instigating the war, weakening the Muslim nation by dividing the Ummah, fabricating self-aggrandizing heresies slandering the Prophet's family and even selling his Muslim critics into slavery in the Byzantine empire.One of Muawiyah's most controversial and enduring legacies was his decision to designate his son Yazid as his successor.According to Shi'a doctrine, this was a clear violation of the treaty he made with Hasan ibn Ali.