Umm Kulthūm bint Ali was the 4th child of Ali and Fatima. Her believed marriage to the Caliph Umar al-Khattab is the cause of much contention between the Sunni and the Shiites. The reason for this is that Caliph Umar al-Khattab is considered to be a disrespected character by the Shi'as and therefore unworthy of the daughter of Ali, Even though, that Umm Kulthum bint Ali had a son from Umar named him Zayd ibn Omar ibn al-Khattab (famously known as Ibnul Khalifatayn; the son of the two Caliphs i.e. Umar and Ali), and a daughter named her Ruqayyah bint Umar