Umar ibn al-Khattab, a companion of the prophet, said:
"One day we were sitting in the company of Allah's Apostle (peace be upon him) 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 Apostle (peace be upon him).

He knelt before him placed his palms on his thighs and said: Muhammad, inform me about Islam.

"The Messenger of Allah (peace be upon him) said: Islam implies that you testify that there is no god but Allah and that Muhammad is the messenger of Allah, 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.

"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.

"He (the Holy Prophet) replied: That you affirm your faith in Allah, in His angels, in His Books, in His Apostles, 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 again said: Inform me about Ihsan.

"He (the Holy Prophet) said: That you worship Allah 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 Holy 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 Holy 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.

"Then he (the inquirer) went on his way but I stayed with him (the Holy Prophet) for a long while. He then, said to me: Umar, do you know who this inquirer was? I replied: Allah and His Apostle knows best. He (the Holy Prophet) remarked: He was Gabriel (the angel). He came to you in order to instruct you in matters of religion."
Sahih Muslim, Book 1:Number 1