Musnad Ahmad 2:285

Muhammad ibn Ja‘far narrated to us, Kahmas narrated to us, from Ibn Buraydah. And Yazid ibn Harun said: Kahmas narrated to us, from Ibn Buraydah, from Yahya ibn Ya‘mar, who heard Ibn ‘Umar say: ‘Umar ibn al-Khattab narrated to me, saying: While we were one day with the Prophet of Allah, may Allah bless him and grant him peace, a man appeared to us with extremely white clothes and extremely black hair. No trace of travel was seen upon him—Yazid said: We did not see upon him any trace of travel—and none of us knew him, until he sat down by the Prophet of Allah, may Allah bless him and grant him peace. He leaned his knees against his knees and placed his palms upon his thighs. Then he said: "O Muhammad, inform me about Islam. What is Islam?" He said: "Islam is that you bear witness that there is no god but Allah and that Muhammad is the Messenger of Allah, and that you establish prayer, give zakat, fast Ramadan, and perform pilgrimage to the House if you are able to find a way to it." He said: "You have spoken the truth." He said: We were amazed at him; he asks him and confirms him. He said: Then he said: "Inform me about faith (iman)." He said: "Faith is that you believe in Allah, His angels, His books, His messengers, the Last Day, and the decree, all of it, its good and its evil." He said: "You have spoken the truth." He said: "Then inform me about excellence (ihsan). What is excellence?" Yazid said: "That you worship Allah as if you see Him, for if you do not see Him, He sees you." He said: "Then inform me about the Hour." He said: "The one questioned about it knows no more than the questioner." He said: "Then inform me about its signs." He said: "That the slave-girl gives birth to her mistress, and that you see the barefoot, naked, sheep-herders competing in the construction of buildings." He said: Then he departed. He said: I remained for a while—Yazid said: for three [days]—then the Messenger of Allah, may Allah bless him and grant him peace, said to me: "O ‘Umar, do you know who the questioner was?" He said: I said: Allah and His Messenger know best. He said: "Indeed, he was Gabriel; he came to you to teach you your religion."