This article focuses too much on specific examples without clearly discussing its abstract general subject. Please help by consulting secondary sources for general statements made about the subject and improving this article by adding more general information.(March 2014)
One day while the Prophet was sitting in the company of some people, (The angel) Gabriel came and asked, "What is faith?" Allah's Apostle replied, 'Faith is to believe in Allah, His angels, (the) meeting with Him, His Apostles, and to believe in Resurrection." Then he further asked, "What is Islam?" Allah's Apostle replied, "To worship Allah Alone and none else, to offer prayers perfectly to pay the compulsory charity (Zakat) and to observe fasts during the month of Ramadan." Then he further asked, "What is Ihsan (perfection)?" Allah's Apostle replied, "To worship Allah as if you see Him, and if you cannot achieve this state of devotion then you must consider that He is looking at you." Then he further asked, "When will the Hour be established?" Allah's Apostle replied, "The answerer has no better knowledge than the questioner. But I will inform you about its portents."
When a slave (lady) gives birth to her master.
When the shepherds of black camels start boasting and competing with others in the construction of tall buildings. And the Hour is one of five things which nobody knows except Allah.
The Prophet then recited: "Verily, with Allah (Alone) is the knowledge of the Hour--." (31. 34) Then that angel (Gabriel) left and the Prophet asked his companions to call him back, but they could not see him. Then the Prophet said, "That was Gabriel who came to teach the people their religion." Abu 'Abdullah said: He (the Prophet) considered all that as a part of faith. Sahih al-Bukhari, 1:2:48
While we were one day sitting with the Messenger of Allah, peace and blessings of Allah upon him, there appeared before us a man dressed in extremely white clothes and with very black hair. No traces of travel were visible on him, and none of us knew him. He sat down close by the Prophet, peace and blessings upon him, rested his knees against his knees and placed his palms on his thighs, and said, O Muhammad! Inform me about Islam." Muhammad said, "Islam is that you should testify that there is no deity save Allah and that Muhammad is His Messenger, that you should perform salah (ritual prayer), pay the zakah (alms), fast during Ramadan, and perform Hajj (pilgrimage) to the House (the Ka'bah at Makkah), 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 Iman (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 (time of the resurrection)." 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 (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." [Muslim]