Sahih Muslim Book 001, Hadith Number 0006.
Chapter : What is Iman and what are its characteristics.
It is narrated on the authority of Abu Huraira that the Messenger of Allah (may peace be upon him) said: Ask me (about matters pertaining to religion), but they (the Companions of the Holy Prophet) were too much overawed out of profound respect for him to ask him (anything). In the meanwhile a man came there, and sat near his knees and said: Messenger of Allah, what al-Islam is?-to which he (the Holy Prophet)replied: You must not associate anything with Allah, and establish prayer, pay the poor-rate (Zakat) and observe (the fasts) of Ramadan. He said: You (have) told the truth. He (again) said: Messenger of Allah, what al-Iman (the faith) is? He (the Holy Prophet) said: That you affirm your faith in Allah, His angels, His Books, His meeting, His Apostles, and that you believe in Resurrection and that you believe in Qadr (Divine Decree) in all its entirety, He (the inquirer) said: You (have) told the truth. He (again) said: Messenger of Allah, what al-Ihsan is? Upon this he (the Holy Prophet) said: (Al-Ihsan implies) that you fear Allah as if you are seeing Him, and though you see Him not, verily He is seeing you. He (the inquirer) said: You(have) told the truth. He (the inquirer) said: When there would be the hour (of Doom)? (Upon this) he (the Holy Prophet said: The one who is being asked about it is no better informed than the inquirer himself. I, however; narrate some of its signs (and these are):when you see a slave(woman)giving birth to her master - that is one of the signs of (Doom); when you see barefooted, naked, deaf and dumb (ignorant and foolish persons) as the rulers of the earth - that is one of the signs of the Doom. And when you see the shepherds of black camels exult in buildings - that is one of the signs of Doom. The (Doom) is one of the five things (wrapped) in the unseen. No one knows them except Allah. Then(the Holy Prophet) recited (the following 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 on morrow 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 stood up an (made his way). The Messenger of Allah (may peace be upon him) said: Bring him back to me. He was searched for, but they (the Companions of the Holy Prophet) could not find him. The Messenger of Allah (may peace be upon him) thereupon said: He was Gabriel and he wanted to teach you (things pertaining to religion) when you did not ask (them yourselves).