Islamic tradition relates that during one of Muhammad's isolated retreats to the mountains, he received his first revelation in the Cave of Hira. Thereafter, he received revelations over a period of twenty-three years. According to hadith and Muslim history, after Muhammad emigrated to Medina and formed an independent Muslim community, he ordered a considerable number of the companions (sahaba) to recite the Qur’an and to learn and teach the laws which were being revealed daily. Companions who engaged in the recitation of the Qur’an were called qurra'. Since most sahaba were unable to read or write, they were ordered to learn from the prisoners-of-war the simple writing of the time. Thus a group of sahaba gradually became literate. As it was initially spoken, the Qur’an was recorded on tab