" Umar said: “One day when we were sitting with the Messenger of God there came unto us a man whose clothes were of exceeding whiteness and whose hair was of exceeding blackness, nor were there any signs of travel upon him, although none of us knew him. He sat down knee unto knee opposite the Prophet, upon whose thighs he placed the palms of his hands, saying: “O Muhammad, tell me what is the surrender (islam)’. The Messenger of God answered him saying: ‘The surrender is to testify that there is no god but God and that Muhammad is God’s Messenger, to perform the prayer, bestow the alms, fast Ramadan and make, if thou canst, the pilgrimage to the Holy House.’ He said: ‘Thou hast spoken truly,’ and we were amazed that having questioned him he should corroborate him. Then he said: ‘Tell me what is faith (iman).’ He answered: ‘To believe in God and His Angels and His Books and His Messengers and the Last Day, and to believe that no good or evil cometh but by His Providence.’ ‘Thou hast spoken truly,’ he said, and then: ‘Tell me what is excellence (ihsan).’ He answered: ‘To worship God as if thou sawest Him, for if thou seest Him not, yet seeth He thee.’ ‘Thou hast spoken truly,’ he said, and then: ‘Tell me of the Hour.’ He answered: ‘The questioned thereof knoweth no better than the questioner.’ He said: ‘Then tell me of its signs.’ He answered: ‘That the slave-girl shall give birth to her mistress; and that those who were but barefoot naked needy herdsmen shall build buildings ever higher and higher.’ Then the stranger went away, and I stayed a while after he had gone; and the Prophet said to me: ‘O ‘Umar, knowest thou the questioner, who he was?’ I said: ‘God and His Messenger know best.’ He said: ‘It was Gabriel. He came unto you to teach you your religion. "

― , Muhammad: His Life Based on the Earliest Sources