The historicity of Muhammad refers to the study of Muhammad as a historical figure and critical examination of sources upon which traditional accounts are based. The earliest Muslim source of information for the life of Muhammad, the Quran, gives very little personal information and its historicity is debated. Prophetic biography, known as sīra, along with attributed records of the words, actions, and the silent approval of Muhammad, known as hadith, survive in the historical works of writers from the third and fourth centuries of the Muslim era (c. 800−1000 CE), and give a great deal of information on Muhammad, but the reliability of this information is very much debated in some academic circles. In addition there are a relatively small number of contemporaneous or near-contemporaneous non-Muslim sources which attest to the existence of Muhammad and are valuable both in themselves and for comparison with Muslim sources. Islamic sources The main Islamic source on Muhammad's life are the Quran and accounts of Muhammad's life based on oral traditions known as sīra and hadith. Quran According to traditional Islamic scholarship, all of the Quran was written down by Muhammad's companions while he was alive (during CE 610–632), but it was primarily an orally related document. Following the death of Muhammad the Quran ceased to be revealed, and companions who had memorized the Quran began to die off (particularly after the Battle of Yamama in 633). Worried that parts of the Quran might be irretrievably lost, senior companion Umar urged Caliph Abu Bakr to order the collection of the pieces of the Quran which had hitherto been scattered among "palm-leaf stalks, thin white stones, ... [and] men who knew it by heart, ..." put them together. Under Caliph Uthman, a committee of five copied the scraps into a single volume, "monitoring the text as they went", resolving disagreements about verses, tracking down a lost verse. This muṣḥaf – that became known as the "Uthmanic codex" – was finished around 650 CE, whereupon Uthman issued an order for all other existing personal and individual copies and dialects of the Quran (known as Ahruf) to be burnt. Modern scholars differ in their assessment of the Quran as a historical source about Muhammad's life. According to the Encyclopedia of Islam, the "Qur'an responds constantly and often candidly to Muhammad's changing historical circumstances and contains a wealth of hidden data that are relevant to the task of