On the authority of Umar ibn al-Khattab (radhiAllahu 'anhoo): one day we were sitting in the company of Allaah's Prophet (sallAllaahu alayhi wa sallam) when there appeared before us a man dressed in pure white clothes, his hair extraordinarily black. There were no signs of travel on him. None amongst us recognized him. At last he sat with the Prophet (sallAllaahu alayhi wa sallam) He knelt before him placed his palms on his thighs and said: Muhammad, inform me about al-Islam. The Messenger of Allaah (sallAllaahu alayhi wa sallam) said: Al-Islam implies that you testify that there is none worthy of worship (in truth) but Allaah and that Muhammad is the messenger of Allaah, and you establish prayer, pay Zakat, observe the fast of Ramadan, and perform pilgrimage to the (House) if you are solvent enough (to bear the expense of) the journey. He (the inquirer) said: You have told the truth. He (Umar ibn al-Khattab) said: It amazed us that he would put the question and then he would himself verify the truth. He (the inquirer) said: Inform me about Iman (faith). He (the Prophet) replied: That you affirm your faith in Allaah, in His angels, in His Books, in His Prophets, in the Day of Judgment, and you affirm your faith in the Divine Decree about good and evil. He (the inquirer) said: You have told the truth. He (the inquirer) again said: Inform me about al-Ihsan (performance of good deeds). He (the Prophet) said: That you worship Allaah as if you are seeing Him, for though you don't see Him, He, verily, sees you. He (the enquirer) again said: Inform me about the hour (of the Doom). He (the Prophet) remarked: One who is asked knows no more than the one who is inquiring (about it). He (the inquirer) said: Tell me some of its indications. He (the Prophet) said: That the slave-girl will give birth to her mistress and master, that you will find barefooted, destitute goat-herds vying with one another in the construction of magnificent buildings. He (the narrator, Umar ibn al-Khattab) said: Then he (the inquirer) went on his way but I stayed with him (the Prophet) for a long while. He then, said to me: Umar, do you know who this inquirer was? I replied: Allaah and His Prophet knows best. He (the Prophet) remarked: He was Jibra'eel (Gabriel the angel). He came to you in order to instruct you in matters of religion.
(Muslim #5; Aboo Daawood #4695; at-Tarmidhee #2610; an-Nisaa'ee 101-97/8; Ibn Majah #63; Imaam Aboo ya'laa in his Mukhtasar #237; Sharus-Sunnah of Imaam al-Baghawee #2)