Obaidullah bin Omar bin al-Khattab

Ubayd Allah ibn Umar ibn al-Khattab was a son of Caliph Umar. His killing of Hormuzan, whom he suspected of involvement in his father's assassination in 644, and his pardon by Caliph Uthman was opposed by Ali, the cousin of the Islamic prophet Muhammad. When Ali became caliph in 656, he refused Ubayd Allah's appeal to uphold Uthman's pardon, prompting Ubayd Allah to defect to Ali's principal enemy, the governor of Syria Mu'awiya ibn Abi Sufyan. The latter made Ubayd Allah a commander of his elite battalion at the Battle of Siffin, where he was slain.