''While we were one day sitting with the Messenger of Allah (peace be upon him), there appeared before us a man dressed in extremely white clothes and with very black hair. No traces of journeying were visible on him, and none of us knew him. He sat down close by the Prophet (peace be upon him), rested his knee against his thighs, and said, 'O Muhammad! Inform me about Islam.'
The Messenger of Allah (peace be upon him) said, 'Islam is that you should testify that there is no deity except Allah and that Muhammad is His Messenger, that you should perform salah, pay the Zakah, fast during Ramadan, and perform Hajj to the House, if you are able to do so.'
The man said, 'You have spoken truly.' We were astonished at his questioning him (the Messenger) and telling him that he was right, but he went on to say, 'Inform me about iman.'
He (the Messenger of Allah) answered, 'It is that you believe in Allah and His angels and His Books and His Messengers and in the Last Day, and in qadar (fate), both in its good and in its evil aspects.' He said, 'You have spoken truly.'
Then he (the man) said, 'Inform me about Ihsan.' He (the Messenger of Allah) answered, 'It is that you should serve Allah as though you could see Him, for though you cannot see Him yet (know that) He sees you.'
He said, 'Inform me about the Hour.' He (the Messenger of Allah) said, 'About that, the one questioned knows no more than the questioner.' So he said, 'Well, inform me about the signs thereof.' He said, 'They are that the slave-girl will give birth to her mistress, that you will see the barefooted, naked, destitute, the herdsmen of the sheep (competing with each other) in raising lofty buildings.' Thereupon the man went of.''
I waited a while, and then he (the Messenger of Allah) said, 'O Umar, do you know who that questioner was?' I replied, 'Allah and His Messenger know better.' He said, 'That was Jibril (the Angel Gabriel). He came to teach you your religion.'
[Muslim]
Commentary Summary
Written commentary compiled by volunteers utilizing Sh. Jamaal Diwan's audio commentary above and English translation of Ibn Daqiq Al-Id's commentary on The Forty Hadith of Imam al-Nawawi.
The Significance of this Hadith
This hadith includes within it all outwardly actions and inwardly beliefs. The sciences of the Sharia’ return back to this hadith due to it encompassing knowledge of the Sunnah. Hence, some of the scholars have termed this hadith the Mother or Core of the Sunnah, just as Surah al-Fatiha has been termed the Mother or Core of the Qur'aan, due to it containing the entire Message of the Qur'an.
This hadith is also known as Hadith Jibreel (Angel Gabriel) (as). The hadith tells the story of Jibreel (as) when he came to the Prophet (sas), asking a set of questions. The Prophet (sas) responded to those questions and then addressed his companions. There is a door called “Baab Jibreel” at Masjid An-Nabawi in Madina. It is known as the door that Jibreel (as) came in to ask the Prophet (sas) these questions.
http://www.40hadithnawawi.com/index.php/the-hadiths/hadith-2