The term 'JERUSALEM's Mosque of Omar' refers to the Dome of the Rock, a shrine located in JERUSALEM. It is an Islamic shrine that was built by the Umayyad Caliph Abd al-Malik ibn Marwan in 691 CE and is considered to be one of the oldest surviving works of Islamic architecture. The shrine is named after the second Rashidun Caliph, Omar ibn al-Khattab, who is said to have prayed there when he conquered JERUSALEM in 637 CE.