narration attributed to Abu Hurairah reports:

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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 One day while we were sitting with the messenger of Allah there appeared before us a man whose clothes were exceedingly white and whose hair was exceedingly black; no signs of journeying were to be seen on him and none of us knew him. He walked up and sat down by the prophet. Resting his knees against his and placing the palms of his hands on his thighs, he said:"O Muhammad, tell me about Islam". The messenger of Allah said: "Islam is to testify that there is no god but Allah and Muhammad is the messenger of Allah, to perform the prayers, to pay the zakat, to fast in Ramadan, and to make the pilgrimage to the House if you are able to do so." He said: "You have spoken rightly", and we were amazed at him asking him and saying that he had spoken rightly. He said: "Then tell me about Iman."He said:"It is to believe in Allah, His angels, His books, His messengers, and the Last Day, and to believe in divine destiny, both the good and the evil thereof." He said: "You have spoken rightly". He said: " Then tell me about Ihsan." He said: "It is to worship Allah as though you are seeing Him, and while you see Him not yet truly He sees you". He said: "Then tell me about the Hour". He said: "The one questioned about it knows no better than the questioner." He said: "Then tell me about its signs." He said: "That the slave-girl will give birth to her mistress and that you will see the barefooted, naked, destitute herdsman competing in constructing lofty buildings." Then he took himself off and I stayed for a time. Then he said: "O Umar, do you know who the questioner was?" I said: "Allah and His messenger know best". He said: "He was Gibreel (Gabriel), who came to you to teach you your religion." (Narrated by Muslim)