Once we were sitting in the company of Messenger of Allah (ﷺ) when there appeared a man dressed in very white clothes and having extraordinary black hair. No signs of fatigue of journey appeared on him and he was known to none of us. He sat down facing the Prophet (ﷺ) leaning his knees against the knees of the Prophet (ﷺ) and placing both of his palms over his two thighs and said, "O Muhammad (ﷺ)! Tell me about Islam". He (ﷺ) replied, "Islam is to testify that none has the right to be worshipped but Allah, and that Muhammad (ﷺ) is the Messenger of Allah; that you observe Salat (prayers), pay Zakat, observe Saum (fasting) of Ramadan and perform Hajj (pilgrimage) of the House, provided you have resources of making journey to it." He replied: "You have spoken the truth." We were surprised to see that he had asked him and confirmed the correctness of the answers. He then enquired: "Tell me about Iman." He (ﷺ) said: "It is to believe in Allah, His angels, His Books, His Messengers and the Last Day and that you believe in preordainment (destiny), its bad and good consequences." He said, "You have spoken the truth." He then enquired: "Tell me about Ihsan." He (ﷺ) said, "It is to worship Allah as if you are seeing Him; and although you do not see Him, He sees you." He enquired: "Inform me about the Hour (i.e., the Day of Resurrection)." He (ﷺ) replied, "I have no more knowledge thereof than you". He said, "Inform me about some of its signs." He (ﷺ) said, "They are - that a bondswoman gives birth to her own master, and that you will find the barefooted, naked, poor shepherds competing one another in the construction of higher buildings." Then he departed. The Messenger of Allah kept silent for a while then he said to me, "O 'Umar! Do you know who the questioner was?" I replied, "Allah and His Messenger know better." The Prophet (ﷺ) said, "He was Jibril (Gabriel); he came to you to teach you your religion."

[Muslim]