such as the Torah and the Gospels. The term mus'haf ('written work') is often used to refer to particular Quranic manuscripts but is also used in the Quran to identify earlier revealed books. History Prophetic era Islamic tradition relates that Muhammad received his first revelation in the Cave of Hira during one of his isolated retreats to the mountains. Thereafter, he received revelations over a period of 23 years. According to hadith and Muslim history, after Muhammad immigrated to Medina and formed an independent Muslim community, he ordered many of his companions to recite the Quran and to learn and teach the laws, which were revealed daily. It is related that some of the Quraysh who were taken prisoners at the Battle of Badr regained their freedom after they had taught some of the Muslims the simple writing of the time. Thus a group of Muslims gradually became literate. As it was initially spoken, the Quran was recorded on tablets, bones, and the wide, flat ends of date palm fronds. Most suras were in use amongst early Muslims since they are mentioned in numerous sayings by both Sunni and Shia sources, relating Muhammad's use of the Quran as a call to Islam, the making of prayer and the manner of recitation. However, the Quran did not exist in book form at the time of Muhammad's death in 632. There is agreement among scholars that Muhammad himself did not write down the revelation. Sahih al-Bukhari narrates Muhammad describing the revelations as, "Sometimes it is (revealed) like the ringing of a bell" and Aisha reported, "I saw the Prophet being inspired Divinely on a very cold day and noticed the sweat dropping from his forehead (as the Inspiration was over)." Muhammad's first revelation, according to the Quran, was accompanied with a vision. The agent of revelation is mentioned as the "one mighty in power," the one who "grew clear to view when he was on the uppermost horizon. Then he drew nigh and came down till he was (distant) two bows' length or even nearer." The Islamic studies scholar Welch states in the Encyclopaedia of Islam that he believes the graphic descriptions of Muhammad's condition at these moments may be regarded as genuine, because he was severely disturbed after these revelations. According to Welch, these seizures would have been seen by those around him as convincing evidence for the superhuman origin of