Musnad Ahmad 460:3

Abu al-Yaman narrated to us, Shu'ayb informed us, he said: Abdullah ibn Abi Husayn narrated to us, Shahr ibn Hawshab narrated to us, from 'Amir or Abu 'Amir or Abu Malik, that the Prophet, peace and blessings of Allah be upon him, while he was sitting in a gathering with his companions, Gabriel, peace be upon him, came to him in a form other than his own, and he (the Prophet) thought he was a man from the Muslims. He greeted him, and he returned the greeting. Then Gabriel placed his hand on the knees of the Prophet, peace and blessings of Allah be upon him, and said to him: "O Messenger of Allah, what is Islam?" He said: "That you submit your face to Allah, and that you bear witness that there is no god but Allah and that Muhammad is His servant and His Messenger, and that you establish prayer, and give Zakat." He said: "If I do that, have I submitted?" He said: "Yes." Then he said: "What is Iman (faith)?" He said: "That you believe in Allah, the Last Day, the angels, the Book, the prophets, death, life after death, Paradise, the Fire, the Reckoning, the Balance, and the Decree, all of it, its good and its evil." He said: "If I do that, have I believed?" He said: "Yes." Then he said: "What is Ihsan (excellence), O Messenger of Allah?" He said: "That you worship Allah as if you see Him, for if you do not see Him, He sees you." He said: "If I do that, have I done excellence?" He said: "Yes." And we would hear the response of the Messenger of Allah, peace and blessings of Allah be upon him, to him, but we would not see the one who was speaking to him, nor would we hear his speech. He said: "So when is the Hour, O Messenger of Allah?" The Messenger of Allah, peace and blessings of Allah be upon him, said: "Glory be to Allah, five things of the Unseen, no one knows them except Allah, the Mighty and Majestic: {Indeed, Allah [alone] has knowledge of the Hour and sends down the rain and knows what is in the wombs. And no soul perceives what it will earn tomorrow, and no soul perceives in what land it will die. Indeed, Allah is Knowing and Acquainted} [Luqman: 34]." The questioner said: "O Messenger of Allah, if you wish, shall I tell you of two signs that will occur before it?" He said: "Tell me." He said: "When you see the slave-woman giving birth to her master, and the people of the buildings competing in building, and the poor, barefoot people becoming the heads of the people." He said: "And who are those, O Messenger of Allah?" He said: "The Arabs." He said: Then he turned away, and when we did not see his path anymore, he said: "Glory be to Allah" — three times — "This is Gabriel, who came to teach the people their religion. By the One in Whose hand is the soul of Muhammad, he has never come to me except that I recognized him, except for this time."