In The Name of Allaah, The Most Merciful, The Bestower of Mercy.
Umar [radiyallaahu-anhu] narrated, “One day while we were sitting with the Messenger of Allaah [sallal-laahu-alayhi-wasallam], a man with very white clothing and very 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”’.
Shaikh Saaleh Aala Ash-Shaikh [hafidhahullaah] said: Some of the Ulamaa named this Hadeeth Ummus Sunnah [Mother of the Sunnah] because just as there is Ummul Qur’aan [the Mother of the Qur’aan, i.e. Surah Al-Faatihah], likewise this hadeeth is Ummus Sunnah because all the Sunnah return [or revolves] round this hadeeth [in general]. In this hadeeth, there’s a clarification regarding the [sound] Aqeedah because Aqeedah is the foundation of the six pillars of Eemaan. In this hadeeth, there is a clarification of the Sharee’ah by way of the five pillars of Islaam. In this hadeeth, there is a mention of the affairs of the unseen, and the mention of Ihsaan necessitates the inclusion of good manners, etiquettes, acts of worship, rectification of the heart and turning to Allaah [sincerely in worship, submission etc]. In this hadeeth, there is a mention of the Hour [i.e. the day of judgement] and some of its minor signs, which are a part of the knowledge of the unseen. [An Excerpt from Sharh Al-Arba’een An-Nawawiyyah. Pages 18-19. Slightly paraphrased]