Saiyidina Umar al Khattab Radhiallahu Anhu narrated :
" One day while we were sitting with Allah's Messenger Sallalahu Alaihi Wassallam, a man suddenly appeared before us, wearing a very white dress and having very black hair, without any signs of journey upon him and none of us knew him.
He approached until he sat before the Prophet Sallalahu Alaihi Wassallam with his knees touching the Prophet's knees and he placed his hands on the Messenger's thighs and asked,
"O Muhammad, inform me about Islam!".
Allah's Messenger Sallalahu Alaihi Wassallam answered, "Islam is to bear witness that none has the right to be worshipped but Allah, and that Muhammad is the Messenger of Allah; to offer the Salat; pay Zakat ; fast during the month of Ramadhan and to make the Hajj ( the pilgrimage to Makkah) if you are able and have the means to make the journey."
The man said," You spoke the truth."
We were surprised at his asking and confirming at the same time.
He then said," Inform me about Iman."
The Prophet Sallalahu Alaihi Wassallam said," Iman is to believe in Allah ; His Angels ; His Books and Messengers ; The Last Day and to believe in the Divine Preordainment of all that is good and evil."
He again said," You spoke the truth."
He said," Inform me about 'Ihsan."
Rasulullah Sallalahu Alaihi Wassallam said," 'Ihsan (perfection) is to worship Allah as if you see Him ; if you can't see Him, surely He sees you."
The man said," Inform me about the Hour (Doomsday)."
The Prophet Sallalahu Alaihi Wassallam said," The one asked has no more knowledge of it than the questioner."
The man then asked," Inform me about it's (Doomsday) signs."
Allah's Messenger Sallalahu Alaihi Wassallam said," (It's signs are) the slave girl will give birth to her mistress and you will see the barefooted, naked, impoverished sheepherders competing with each other in raising tall buildings."
Then the stranger left.
The Prophet Sallalahu Alaihi Wassallam remained seated for a while , then he asked me, " O 'Umar, do you know who the questioner was?"
I said," Allah and His Messenger know best."
He said," That was Jibrael, he came to teach you your religion."
Hadith Sahih by Muslim.