It was narrated on the authority of Umar (may Allah be pleased with him), who said:
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 off. 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]
This hadith clears the meaning and let us understand Islam, Iman, Ihsan because it includes outward actions and inwardly beliefs.
Some scholars have termed this hadith the Mother or Core of the Sunnah. Because, the sciences of the Sharia’ return back to it, due to its encompassing knowledge of the Sunnah.