In about AD 650, Uthman began noticing slight differences in pronunciation of the Quran as Islam expanded beyond the Arabian Peninsula into Persia, the Levant, and North Africa. In order to preserve the sanctity of the text, he ordered a committee headed by Zayd ibn Thabit to use caliph Abu Bakr's copy and prepare a standard copy of the Qur'an. Thus, within 20 years of Muhammad's death, the Quran was committed to written form. That text became the model from which copies were made and promulgated throughout the urban centers of the Muslim world, and other versions are believed to have been destroyed.