Sahih Bukhari Volume 6, Book 60, Hadith Number 300.

Narrated By Abu Huraira : One day while Allah’s Apostle was sitting with the people, a man came to him walking and said, “O Allah’s Apostle. What is Belief?” The Prophet said, “Belief is to believe in Allah, His Angels, His Books, His Apostles, and the meeting with Him, and to believe in the Resurrection.” The man asked, “O Allah’s Apostle What is Islam?” The Prophet replied, “Islam is to worship Allah and not worship anything besides Him, to offer prayers perfectly, to pay the (compulsory) charity i.e. Zakat and to fast the month of Ramadan.” The man again asked, “O Allah’s Apostle What is Ihsan (i.e. perfection or Benevolence)?” The Prophet said, “Ihsan is to worship Allah as if you see Him, and if you do not achieve this state of devotion, then (take it for granted that) Allah sees you.” The man further asked, “O Allah’s Apostle When will the Hour be established?”

The Prophet replied, “The one who is asked about it does not know more than the questioner does, but I will describe to you its portents. When the lady slave gives birth to her mistress, that will be of its portents; when the bare-footed naked people become the chiefs of the people, that will be of its portents. The Hour is one of five things which nobody knows except Allah. Verily, the knowledge of the Hour is with Allah (alone). He sends down the rain, and knows that which is in the wombs.” (31.34) Then the man left. The Prophet said, “Call him back to me.” They went to call him back but could not see him. The Prophet said, “That was Gabriel who came to teach the people their religion.” (See Hadith No.47 Vol.1)