Umar ibn Khattab (Allah be well pleased with him) said:
As we sat one day with the Messenger of Allah (Allah bless him and give him peace), a man with pure white clothing and jet black hair came to us, without a trace of travelling upon him, though none of us knew him.
He sat down before the Prophet (Allah bless him and give him peace) bracing his knees against his, and resting his hands on his legs, said:
‘Muhammad, tell me about Islam.’
The Messenger of Allah (Allah bless him and give him peace) said:
‘Islam is to testify that there is no god but Allah and that Muhammad is the Messenger of Allah, and to perform the prayer, give zakat, fast in Ramadan, and perform pilgrimage to the House if you can find a way.’
The man said:
‘You have spoken the truth,’
and we were surprised that he should ask and then confirm the answer.
Then the man said:
‘Tell me about true faith (iman),’
and the Prophet (Allah bless him and give him peace) answered:
‘It is to believe in Allah, His angels, His revealed Books, His messengers, the Resurrection Day, and in destiny, its good and evil.’
The man answered:
‘You have spoken the truth.’
‘Now tell me about the perfection of faith (ihsan),’
and the Prophet (Allah bless him and give him peace) answered:
‘It is to worship Allah as if you see Him, and if you see Him not, He nevertheless sees you.’
‘Tell me of the Hour.’
said the visitor, and he was told:
‘The one questioned knows no more about it than the questioner.’
The man said,
‘Then tell me of its signs,’
and the Prophet (Allah bless him and give him peace) replied:
‘That the slave woman shall give birth to her mistress, and that you shall see barefoot, naked, penniless shepherds competing in constructing high buildings.’
Then the visitor left.
I waited a while, and the Prophet (Allah bless him and give him peace) said to me,
‘Do you know, Umar, who was the questioner?’
and I replied,
‘Allah and His messenger know best.’
He said,
‘It was Jibreel, who came to you to teach you your religion.’
(Sahih Muslim)