Sahih Muslim, Chapter: Faith (Kitab Al Iman)

One day the Messenger of Allah (may peace be upon him) appeared before the public that a man came to him and said: Prophet of Allah, (tell me) what is Iman. Upon this he (the Holy Prophet) replied: That you affirm your faith in Allah, His angels, His Books, His meeting, His Messengers and that you affirm your faith in the Resurrection hereafter. He (again) said: Messenger of Allah, (tell me) what does al-Islam signify. He (the Holy Prophet) replied: Al-Islam signifies that you worship Allah and do not associate anything with Him and you establish obligatory prayer and you pay the obligatory poor-rate (Zakat) and you observe the fast of Ramadan. He (the inquirer) again said: Messenger of Allah, what does al-Ihsan imply? He (the Holy Prophet) replied: That you worship Allah as if you are seeing Him, and in case you fail to see Him, then observe prayer (with this idea in your mind) that (at least) He is seeing you. He (the inquirer) again said: Messenger of Allah, when would there be the hour (of Doom)? He (the Holy Prophet) replied: The one who is asked about it is no better informed than the inquirer. I, however, narrate some of its signs (and these are): when the slave girl will give birth to he master, when the naked, barefooted would become the chiefs of the people, these are some of the signs of (Doom). (Moreover) when the shepherds of the black (camels) would exult themselves in buildings, this is one of the signs of (Doom). (Doom) is one of the five (happenings wrapped in the unseen) which no one knows but Allah. Then he (the Messenger of Allah) recited (the verse):"Verily Allah! with Him alone is the knowledge of the hour and He it is Who sends (down the rain) and knows that which is in the wombs and no person knows whatsoever he shall earn tomorrow, and a person knows not in whatsoever land he shall die. Verily Allah is Knowing, Aware. He (the narrator, Abu Huraira) said: Then the person turned back and went away. The Messenger of Allah (may peace be upon him) said: Bring that man back to me. They (the Companions of the Holy Prophet present there) went to bring him back, but they saw nothing there. Upon this the Messenger of Allah remarked: he was Gabriel, who came to teach the people their religion.