(37) بَابُ سُؤَالِ جِبْرِيلَ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ عَنِ الإِيمَانِ وَالإِسْلاَمِ وَالإِحْسَانِ وَعِلْمِ السَّاعَةِ
Narrated Abu Huraira:
One day while the Prophet (ﷺ) was sitting in the company of some people, (The angel) Gabriel came and asked, "What is faith?" Allah's Messenger (ﷺ) replied, 'Faith is to believe in Allah, His angels, (the) meeting withHim, His Apostles, and to believe in Resurrection." Then he further asked, "What is Islam?" Allah's Messenger (ﷺ) 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 Messenger (ﷺ) 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 Messenger (ﷺ) replied, "The answerer has no better knowledge than the questioner. But I will inform you about its portents.1. When a slave (lady) gives birth to her master.2. When the shepherds of black camels start boasting and competing with others in the construction of higher buildings. And the Hour is one of five things which nobody knows except Allah.The Prophet (ﷺ) then recited: "Verily, with Allah (Alone) is the knowledgeof the Hour--." (31. 34) Then that man (Gabriel) left and the Prophet (ﷺ) asked his companions to call him back, but they could not see him. Then the Prophet (ﷺ) 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.
Reference: Sahih al-Bukhari 50
In-book reference: Book 2, Hadith 43
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 1, Book 2, Hadith 48