When the Persian capital of Ctesiphon in province of Khvârvarân (today known as Iraq) fell to the Muslims during the Islamic conquest of Persia in 637 under the military command of Sa'ad ibn Abi Waqqas during the caliphate of Umar, the palaces and their archives were burned. According to a 17th-century account cited by Georgie Zeidan, the Arab Commander Sa'ad ibn Abi Waqqas wrote to Caliph Umar ibn al-Khattab asking what should be done with the books at Ctesiphon. Umar wrote back: ""If the books contradict the Qur'an, they are blasphemous. On the other hand, if they are in agreement, they are not needed, as for us Qur'an is sufficient."" Thus, the huge library was destroyed and the books, the product of the generations of Persian scientists and scholars were thrown into fire or the Euphrates. However, it is doubted whether libraries where actually burned or desecrated. Nearly 40,000 captured Persian noblemen were taken as slaves and sold in Arabia. The Arabs called the Persians 'Ajam' meaning foreign. The first voice of protest came from Piruz Nahavandi, an enslaved Persian artisan, who assassinated Umar. When the city of Estakhr in the south, a Zoroastrian religious center, put up stiff resistance against the Arab invaders, 40,000 residents were slaughtered or hanged.