The Dome of the rock is one of the oldest standing monuments of Islamic architecture.
The Dome of the Rock is the oldest standing monument of Islamic architecture, as shown in Elad 1995, Grabar 2005, Grabar 2006, and Grabar and Kedar 2009. Grabar 2006 and Elad 1995 also show that it was built on the site of the ruined Second Jewish Temple by orders of the Umayyad caliph ʿAbd al-Malik b. Marwan (r. 685–705 CE). Grabar 1959 discusses the construction, which was completed in 691–692 CE, although Blair 1992 shows that some believe 692 to be the starting point of the construction. Source: Dome of the Rock.
However, the mosques of the Prophet at Medina built in 622 C. E., the Mosque of Kufa in Iraq, the Mosque of Basra in Iraq, the Mosque of Amru, Fustat, Egypt predate the Dome of the Rock.
The House and Mosque of the Prophet at Medina: The Islamic prototype of both religious and residential architecture, built in 622 C. E. (1 A.H.or After Hegire, the emigration of the Prophet to Medina), and enlarged several times in the first century of Islam, the most important of which is the enlargement of al-Walid, the Umayyad caliph, in 707. The Early Congregational Mosques of the Amsar: Misr: Name of early Islamic garrison towns, founded in the conquered regions. The most famous amsar (pl. of misr) were Busra and Kufa in Iraq, Fustat in Egypt, and Qayrawan in Tunisia. These settlements grew from informal encampments around a central mosque to capital cities within the first century Hegire (7th-8th century C. E.). Source: The Architecture of the Early Mosques