The city fell to the Rashidun Caliphate in July 633, after a fiercely fought siege. The Arabs retained the name (''Fīrūz Shābūr'') for the surrounding district, but the town itself became known as '''Anbar''' (Middle Persian word for "granary" or "storehouse") from the granaries in its citadel, a name that had appeared already during the 6th century. According to Baladhuri, the third mosque to be built in Iraq was erected in the city by Sa'd ibn Abi Waqqas. Ibn Abi Waqqas initially considered Anbar as a candidate for the location of one of the first Muslim garrison towns, but the fever and fleas endemic in the area persuaded him otherwise.