Umar [radiyallaahu-anhu] narrated: ”One day while we were sitting with the Messenger of Allaah [sallal-laahu-alayhi-wasallam], a man with extremely white clothing and extremely black hair came to us. There were no signs of travel on him and none of us knew him. He [came along] until he sat next to the Prophet [sallal-laahu-alayhi-wasallam]. Then he placed his knees against the knees of the Prophet [sallal laahu-alayhi-wasallam] and placed his hand on his thighs. Then he said: ”O Muhammad, tell me about Islam.” The Messenger of Allaah [sallal laahu alayhi-wasallam] said, ‘Islaam is to testify that there is none worthy of worship except Allaah and that Muhammad is the Messenger of Allah; to establish the prayers, pay the zakat, fast [the month of] Ramadan, and to make pilgrimage to the House if you have the means to do so.’ He said, ‘You have spoken truthfully.’ We were amazed that he asked the question and then said that he [i.e. the Prophet] had spoken truthfully.
He then said, ‘Tell me about Imaan.’ He [the Messenger replied], ‘It is to believe in Allaah, His angles, His books, His Messengers, the Last Day and to believe in the divine decree, [both] the good and the evil thereof.’ He said, ‘You have spoken truthfully.’
He said, ‘Tell me about al-Ihsaan.’  He [the Prophet] replied, ‘It is that you worship Allaah as if you see Him, and even though you do not see Him, [you know] He sees you.’ He said, ‘Tell me about [the appearance of] the Hour [i.e. the day of judgement.’ He [the Prophet] replied, ‘The one being asked does not know more than the one asking.’ He said, ‘Tell me about its signs.’ He [the Prophet] replied, ‘The slave girl shall give birth to her master, and you will see the barefooted, scantily-clothed shepherds competing in constructing lofty buildings.’ Then he [i.e. the man] went away. I [i.e. Umar] stayed for a while [i.e. days]; then he [the Prophet] said, ‘O Umar! Do you know who the questioner was?’ I said, ‘Allaah and His Messenger know best.’ He said, ‘It was Gabriel [i.e. angel Gibreel] who came to teach you your religion.”’