When he was 40, Muhammad felt himself selected by God to be the Arab prophet of true religion. The Arabs, unlike other nations, had hitherto had no prophet. In the cave of Mt. Hira, N of Mecca, he had a vision in which he was commanded to preach. Thereafter throughout his life he continued to have revelations, many of which were collected and recorded in the Qur'an. His fundamental teachings were: there is one God; people must in all things submit to Him; in this world nations have been amply punished for rejecting God's prophets, and heaven and hell are waiting for the present generation; the world will come to an end with a great judgment. He included as religious duties frequent prayer and almsgiving, and he forbade usury.