Musnad Ahmad 30:2374

Isma'il narrated to us, Abu Hayyan narrated to us, from Abu Zur'ah ibn 'Amr ibn Jarir, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) was one day appearing before the people, when a man came to him and said: "O Messenger of Allah, what is faith (Iman)?" He said: "Faith is that you believe in Allah, His angels, His Book, the meeting with Him, His messengers, and that you believe in the final resurrection." He said: "O Messenger of Allah, what is Islam?" He said: "Islam is that you worship Allah, associating nothing with Him, establish the prescribed prayer, give the obligatory zakat, and fast Ramadan." He said: "O Messenger of Allah, what is excellence (Ihsan)?" He said: "That you worship Allah as if you see Him, for if you do not see Him, He sees you." He said: "O Messenger of Allah, when is the Hour?" He said: "The one questioned about it is no more knowledgeable than the questioner, but I will tell you of its signs: when the slave-girl gives birth to her mistress, that is one of its signs; when the barefoot, naked ones become the heads of the people, that is one of its signs; and when the shepherds of livestock compete in the construction of buildings, that is one of its signs. It is among five things that none knows except Allah." Then the Messenger of Allah (peace and blessings of Allah be upon him) recited this verse: "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] Then the man turned away, and the Messenger of Allah (peace and blessings of Allah be upon him) said: "Bring the man back to me." They went to bring him back, but they saw nothing. He said: "This was Gabriel; he came to teach the people their religion."