Volume 1, Book 2, Number 47 :

Narrated by Abu Huraira
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. 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 then recited: "Verily, with Allah (Alone) is the
knowledge of the Hour--." (31. 34) Then that man
(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.

Volume 1, Book 2, Number 47 :sahih bukhari