I thought this hadith was worth expanding on because it provides a very succinct description of Islamic beliefs. The tradition is known as the "Hadith of Jibril," who is also known as the angel Gabriel, and it makes up part of the first hadith in the first book of the first chapter of the sahih ahadith collection by Muslim.
A narration attributed to Umar reports:
While we were one day sitting with the Messenger of Allah, sallallahu 'alayhi wasallam ["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, sallallahu 'alayhi wasallam, rested his knee against his thighs, and said, "O Muhammad! Inform me about Islam." Said the Messenger of Allah, sallallahu 'alayhi wasallam, "Islam is that you should testify that there is no deity save Allah and that Muhammad is His Messenger, that you should perform salah, pay the zakah, fast during Ramadan, and perform pilgrimage to the House [i.e., the Ka'ba], if you can find a way to it (or find the means for making the journey to it)." Said he (the man), "You have spoken truly."
We were astonished at his thus questioning him and telling him that he was right, but he went on to say, "Inform me about Emaan (faith)." 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 fate (qadar), 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 He sees you." He said, "Inform me about the Hour." He (the Messenger of Allah) said, "The one questioned knows no more than the questioner." So he said, "Well, inform me about the signs thereof (i.e. of its coming)." Said he, "They are that the slave-girl will give birth to her mistress, that you will see the barefooted ones, the naked, the 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. He came to teach you your religion."