The negative view of the Umayyads held by Shias is briefly expressed in the Shi'a book "Sulh al-Hasan". According to Shia hadiths, which are not considered authentic by Sunnis, Ali described them as the worst Fitna. In Shia sources, the Umayyad Caliphate is widely described as "tyrannical, anti-Islamic and godless". Shias point out that the founder of the dynasty, Muawiyah, declared himself a caliph in 657 and went to war against Muhammad's son-in-law and cousin, the ruling caliph Ali, clashing at the Battle of Siffin. Muawiyah also declared his son, Yazid, as his successor in breach of a treaty with Hassan, Muhammad's grandson. Another of Muhammad's grandsons, Husayn ibn Ali, would be killed by Yazid in the Battle of Karbala. Further Shia Imams, such as Muhammad's great-grandson, Ali ibn Husayn Zayn al-Abidin would be killed at the hands of ruling Umayyad caliphs.