It is told by 'Umar ibn al-Khattâb, through his son 'Abdullâh (may Allah be pleased with them both).
Imam Muslim placed it first in the Book of Faith of his Sahîh.
Al-Bukhârî also reports it, from Abû Hurayra (may Allah be pleased with him), with some differences in wording.
'Umar says: “A man appeared before us, his clothes extremely white, his hair extremely black.”
“No trace of travel could be seen on him, and none of us knew him.”
That is the puzzle: a stranger must have travelled to get there. So where was the dust of the road?
He went up to the Prophet ﷺ and sat down with him.
“He rested his knees against his knees, and placed his palms on his thighs.”
Then he said: “O Muhammad, tell me about Islâm.”
He asked four questions: about Islâm, about Îmân, about Ihsân, and about the Hour.
After the answers on Islâm and on Îmân, he said: “You have told the truth.”
'Umar says: “We were amazed at him: he asks him, and then confirms what he says.”
A student does not tell his teacher “you are right”. This questioner already knew the answers.
Then he went away. 'Umar says: “I stayed a long while.”
In al-Bukhârî's narration, the Prophet ﷺ said: “Bring him back.” They looked, and saw nothing.
In at-Tirmidhî's narration, the Prophet ﷺ met 'Umar three days later.
He asked: “'Umar, do you know who the questioner was?” 'Umar said: “Allah and His Messenger know best.”
“It was Jibrîl. He came to you to teach you your religion.”
Jibrîl (peace be upon him), the angel who brings down the revelation, had come in the form of a man.
Notice: he says “your religion”. So Islâm, Îmân and Ihsân together make up the religion.
The scholars call them the three levels of the religion. The next chapters take them one by one.
One day, a man came to the Prophet ﷺ while he sat with his Companions. None of them knew him.
His questions would sum up the whole religion.
A stranger asked four questions, and the Companions went home with their whole religion.
Tonight, tell this hadith to someone at home, in your own words, before you go to sleep.